Discovery and Reconstruction of the Remains of the Beacon-Equipped Hollow Enemy Towers along the Ming Great Wall

Abstract

Hollow Enemy Towers, as iconic structures of the Ming Great Wall, are renowned for their roles in defense surveillance, weapon storage, and firearm operation. Recent studies have indicated that certain Hollow Enemy Towers along the Ji Town section of the Ming Great Wall also serve the function of Beacon Towers for beacon signaling. However, previous studies have not definitively determined if these towers were distinctively marked, nor have they provided a comprehensive account of their current distribution and original historical appearance. This paper initially examined the historical documentation of white lime markings employed on the outer walls of certain Hollow Enemy Towers, which served as Beacon Towers during the middle and late Ming periods. Utilizing multidisciplinary methodologies, this research identified remains of lime markings of the Beacon-Equipped Hollow Enemy Towers along the Ji Town section of the Ming Great Wall, illustrating their extensive distribution. We analyzed the material composition and construction techniques of the lime mortar. This analysis clarifies the scope of lime plastering on the exterior walls of these towers and offers a point of reference for restoring their original historical appearance. The results make a significant supplement to the types of signaling structures on the Great Wall, enriching existing understanding of the original appearance of the Great Wall’s historical landscape.

1. Introduction

Signal transmission has consistently held a pivotal role in early warning systems of ancient militaries. In 440 BC, the Peloponnesians alerted Athens of an impending enemy attack by signaling with lighthouses [1]. On the renowned triumphal column of Emperors Trajan (53–117 AD) and Marcus Aurelius (121–180 AD), soldiers of the Roman Empire utilized burning torches from defensive towers to indicate the imminent arrival of barbarians [2] (as shown in Figure 1). The architectural style depicted on Trajan’s column may have been further applied in the stone watchtowers of Hadrian’s Wall [3]. Studies indicate that the watchtowers, along with outpost fortifications, small forts, and independent watchtowers, collectively comprised the warning and signaling system of Hadrian’s Wall, which largely relied on visual signals, particularly the use of fire signals for intelligence transmission [4]. In the medieval period, the Byzantines and Venetians established a long-distance communication network through towers. For instance, the remains of a watchtower on the summit of Acrocorinth in Greece are believed to have been constructed in the 9th to 10th centuries [5], as well as two medieval towers named Pyrgari along the eastern coast of Attica, a historical region of Greece [6]. These towers were typically made of stone and bound together with mortar. After the late 14th century, Christian nations along the Mediterranean coast built a large number of specialized guard posts, such as the cylindrical stone watchtowers known as viglae located on the southern part of Chios Island, aimed at preventing attacks from pirates, neighboring Ottomans, and Western armies [7]. If any war or pirate ships were detected, the guards stationed in the watchtowers would raise the alarm. If it was nighttime, they would use lanterns to transmit warnings [7].

In ancient China, a structure analogous to the defensive towers and watchtowers of ancient Europe was referred to as the Beacon Tower. It played a crucial role as a means of transmitting military early warning information. The utilization of Beacon Towers originated during the Warring States period, was formalized in the Qin and Han dynasties, and reached its peak development during the Ming Dynasty. In practical terms, when faced with an enemy alert, the Beacon Tower and its ancillary facilities would deploy cannons and hoist flags in daylight, while substituting flags with fire lights at night [8]. This method effectively conveyed signals from one tower to the next. In such scenarios, cavalry and infantry stationed at different camps could promptly assemble at the designated site in response to the alert to repel the attack [9].

In order to defend against the rapid attacks of ancient Mongolian nomadic tribes, the Ming Dynasty constructed the Great Wall along the northern border, extending from Hushan in Liaoning in the east to Jiayuguan in the west. Additionally, within a broader area extending into the interior of the country, eleven major military towns were gradually established from east to west, using the Great Wall as the boundary. Ji Town (蓟镇), one of the eleven important military towns on the border of the Mid-late Ming Dynasty, extends about 900 km from Shanhaiguan in the east to Shitang Road Qiliankou in the west [10] (as shown in Figure 2). Other military towns include Liaodong Town (辽东镇), Xuanfu Town (宣府镇), Chang Town (昌镇), Datong Town (大同镇), Shanxi Town (山西镇), Zhenbao Town (真保镇), Yansui Town (延绥镇), Ningxia Town (宁夏镇), Guyuan Town (固原镇), and Gansu Town (甘肃镇) [11,12]. Geographically, Ji Town served as the northeastern barrier of the capital “Jingshi” (京师) of the Ming Dynasty. Within existing knowledge, along the Ming Ji Town Great Wall, the Beacon Towers designed for transmitting early warnings were generally solid constructions consisting of Earth cores enclosed in brick and stone [13] (as shown in Figure 3 and Figure 4a).

During the Longqing and Wanli eras (1567–1620), the Hollow Enemy Tower (空心敌台), a significant military architectural structure on the Great Wall during the Ming Dynasty, was constructed extensively under the leadership of Qi Jiguang, the general of Ji Town. These towers were commonly constructed using a combination of brick and wood, or solely brick, incorporating a stone base at the lower section, one or two hollow levels in the middle with arrow silts (箭窗) on all four sides. On the tops of the Hollow Enemy Towers, there were usually small buildings known as Pufang (铺房). These structures were used by border soldiers for various activities, observing enemy movements, and taking shelter from the wind and rain. The Pufang typically has one or three bays, and their roofs are designed in the gable-style (硬山式) or hip-and-gable style (歇山式). The tops of the Enemy Towers are surrounded by crenelated wall (垛口墙) on all four sides (as shown in Figure 3 and Figure 4b) [14,15]. Based on contemporary academic research [16], the Hollow Enemy Tower fulfilled various roles, such as troop garrisoning, weapon storage, lookout duties, and firearm operation. As documented in “The Chronicles of Four Towns and Three Passes” (四镇三关志) [17], penned in the fourth year of the Wanli era (1576), a cumulative count of 1093 Hollow Enemy Towers was erected along the Great Wall of Ji Town starting from the second year of the Longqing era (1568). The emergence of these structures substantially bolstered the defensive capacity of the Great Wall, establishing them as emblematic architectural elements of the Great Wall in China.

Figure 3. The “Map of the Ming Ji Town Great Wall” (1583) depicts the Beacon Towers and Hollow Enemy Towers of Ji Town [16].

Figure 3. The “Map of the Ming Ji Town Great Wall” (1583) depicts the Beacon Towers and Hollow Enemy Towers of Ji Town [16].

Figure 4. (a) The Perspective View of Naziyu No. 05 Beacon Tower in Qinhuangdao City, Hebei Province, based on the 3D point cloud model. (b) The Hollow Enemy Towers along the Great Wall at Dongjiakou Village, Qinhuangdao City, Hebei Province.

Figure 4. (a) The Perspective View of Naziyu No. 05 Beacon Tower in Qinhuangdao City, Hebei Province, based on the 3D point cloud model. (b) The Hollow Enemy Towers along the Great Wall at Dongjiakou Village, Qinhuangdao City, Hebei Province.

According to historical records found in “The Chronicles of Four Towns and Three Passes”, prior to the fourth year of the Wanli era (1576) during the Ming Dynasty, the Ji Town had 569 Beacon Towers, with the majority situated away from the Great Wall [17] (as shown in Figure 3). The quantity of these towers was lower in comparison to the 1093 Hollow Enemy Towers constructed along the wall within the same timeframe. As indicated in “Huang Ming Jing Shi Wen Bian” (皇明经世文编) [18], the Ji Town was esteemed as the foremost of the Ming dynasty’s nine border defenses, underscoring its crucial military strategic significance. How could the beacon transmission system operate efficiently with fewer Beacon Towers, achieving what is stated in the “Practical Records of Military Training” (练兵实纪), that “the Great Wall of Ji Town, extending over 1000 km in a convoluted pattern, could be traversed in just 6 h” [19]? Was it possible that the numerous Hollow Enemy Towers built along the Great Wall of Ji Town also served as Beacon Towers? This has been an important and ongoing question of interest within the academic community, as the following paragraph demonstrates.

Previous studies have initiated an investigation into the potential utilization of Hollow Enemy Towers in Ji Town for beacon signals, drawing upon historical records as an initial source of evidence. Based on Qi Jiguang’s discourse in “The Practical Records of Military Training” regarding the Hollow Enemy Tower occasionally serving as Beacon Towers [19], Zhao argues that the garrison at the Hollow Enemy Tower also had the duty to relay beacon warnings [20]. Subsequently, Xing mention that during the period between Longqing and Wanli (1567–1620), Qi Jiguang oversaw the construction of Hollow Enemy Towers in Ji Town, where historical records confirm that solid Beacon Towers are manned for surveillance where they exist, while Hollow Enemy Towers serve as Beacon Towers in their absence [21]. Tang similarly cites historical records of Hollow Enemy Towers serving as Beacon Towers, suggesting they were habitually co-located with Beacon Towers to fulfill surveillance and communication roles [16].

Through the analysis of historical records, prior research has indicated that the Hollow Enemy Towers in Ji Town, under the leadership of Qi Jiguang, were utilized not only for garrisoning, storage, external surveillance, and weapon firing, but many also functioned as Beacon Towers for transmitting signal fire early warnings. This statement establishes the groundwork for subsequent thorough investigation. However, there is still a lack of clarity regarding the differentiation between Hollow Enemy Towers serving as Beacon Towers and regular Enemy Towers. Additionally, it is uncertain whether any architectural remnants of these Beacon-Equipped Hollow Enemy Towers are extant today, as current research has not addressed these specific aspects.

1.1. The Gap between the Explicit Records and Architectural Remains Research

By examining pertinent historical records (as shown in

Table 1. Historical documentation of Beacon-Equipped Hollow Enemy Towers and their marking practices in Ji Town.

Literature Sources	Historical Documents about the Hollow Enemy Towers Responsible for Beacon Transmission	The Practice of Marking the Beacon-Equipped Hollow Enemy Towers as Recorded in Historical Documents
“Practical Record of Military Training” (练兵实纪)	“Where there is no Hollow Enemy Tower, the original solid Beacon Tower is used to carry out the beacon transmission function. Where a Hollow Enemy Tower was built within 160 m, the Hollow Enemy Tower was used as the Beacon Tower.”	None
“Chronicles of Four Towns and Three Passes” (四镇三关志)	“If the original solid Beacon Towers were insufficient, Hollow Enemy Towers were used as replacements and specially marked.”	Special mark
“Imperial Ming Legal Records” (皇明世法录)	“Where the Hollow Enemy Tower to replace the solid Beacon Tower for signal transmission, the four sides of the crenelated wall and the waist wall, with lime white. The crenelated walls of the original solid Beacon Towers are also whitened with lime to unify the markings.”	The four sides of the crenelated wall and the waist wall, with lime white

), the depictions of Hollow Enemy Towers with the beacon function distinctly labeled have progressed from ambiguous to precise. In “The Practical Record of Military Training”, Qi Jiguang introduced the concept of repurposing Hollow Enemy Towers along the Ji Town Great Wall as Beacon Towers for signal transmission [19]. However, he did not explicitly outline the necessity of special markings for these towers. Later, in “The Chronicles of Four Towns and Three Passes”, it was observed that Hollow Enemy Towers, if used as substitutes for Beacon Towers, ought to be identified [17]. However, the specific methods for marking them were not elaborated upon. In the “Imperial Ming Legal Records” (皇明世法录), Qi Jiguang explicitly mandated the whitewashing of lime on the four sides of the crenelations and the parapet walls of Hollow Enemy Towers responsible for beacon transmission [22]. Likewise, the crenelated walls of the current Beacon Towers were also painted white to guarantee consistency. The historical documents do not provide a clear definition of the specific scope encompassed by the “parapet walls of the tower body” of the Hollow Enemy Towers.

Due to the lack of adequate academic attention to these records, there has been a persistent gap in the understanding of the original appearance and the current state of the Beacon-Equipped Hollow Enemy Towers along the Ming Ji Town Great Wall. Key issues persist without resolution, including the current count of extant towers, their dispersion along the entirety of the Great Wall, the materials and methodologies employed in the application of lime plaster on the outer walls, and the historical authentic appearance of these towers. Neither archaeological reports nor field survey data have yet addressed these issues, highlighting the urgent need for systematic and comprehensive research. This paper also aims to address the key issue mentioned in the previous section.

1.2. Research Objectives

Building upon the documentary analysis of the special marking practices employed on the Beacon-Equipped Hollow Enemy Towers of Ji Town’s segment of the Ming Great Wall, the aim of this study is to comprehensively utilize multidisciplinary techniques from Geographic Information Science, Surveying and Mapping, Computer Science, Archaeology, Materials Science, History, and Architecture to conduct a systematic investigation of Beacon-Equipped Hollow Enemy Towers along the Ming Ji Town Great Wall (as shown in Figure 5). The primary research objectives are outlined as follows:

(1) The identification and screening of plaster marks on the Beacon-Equipped Hollow Enemy Towers of Ming Ji Town Great Wall. This study seeks to examine and verify the physical remnants of plaster marks that were utilized on Beacon-Equipped Hollow Enemy Towers along the entire length in Ji Town, with the objective of elucidating their distribution. This is crucial for the immediate protective conservation of these artifacts.

(2) Study of the plastering materials and construction techniques employed on the exterior walls of the Beacon-Equipped Hollow Enemy Towers in Ji Town. This study aims to conduct a systematic analysis of the material composition and application techniques of the plaster utilized on the exterior walls of these towers, which serve as crucial identifying characteristics. The results will serve as a scientific reference for the protective restoration of plaster mortar.

(3) Reconstruction of the historical appearance of the Beacon-Equipped Hollow Enemy Towers in Ji Town. By integrating historical documents, images, and physical architectural remains, this study aims to reconstruct the original appearance of the Beacon-Equipped Hollow Enemy towers in Ji Town, revealing the architectural originality of these obscured structures of the Ming Great Wall (as shown in Figure 5).

2. Materials and Methods

2.1. Analysis of Beacon Tower and Enemy Tower Density along Entire Ming Great Wall

Utilizing the point density analysis tool within the ArcGIS Pro software (3.0), this research assesses the spatial distribution density of Beacon Towers and Enemy Towers spanning the entirety of the Ming Great Wall. The macro-level analysis of the physical remains confirms the potential dual functionality of the Hollow Enemy Towers in Ji Town, suggesting that they may have served not only as defensive structures but also as Beacon Towers for signal transmission. The point density analysis tool computes the density of point features in the vicinity of each output raster cell. The main principle entails the delineation of a neighborhood surrounding the centroid of each cell, followed by the division of the aggregate points within this neighborhood by its corresponding area. For the initial parameter configuration, the point features provided consist of the geographical coordinates of Beacon Towers and Enemy Towers spanning the length of the Great Wall, sourced from the Chinese Great Wall resource survey in 2007 to 2009. The spatial reference utilized is the WGS 1984 geographic coordinate system.

2.2. Rapid Survey of Heritage Information on a Large Geographical Scale

To comprehensively document the 3D intricacies of the Ming Great Wall remnants and methodically reveal the concealed architectural features, the research team commenced the development of the “Comprehensive 3D Image Database of the Ming Great Wall” in 2018 [23]. The particular implementation approach entails utilizing low-altitude drone photogrammetry from a minimum of three directions, at a relative altitude of 30–50 m above the wall line. This method involves capturing images of the wall, Enemy Towers, and the surrounding micro-terrain. This results in a continuous high-definition image dataset with an overlap rate of at least 70%, enabling the extraction of 3D data on the structures of the Great Wall (as shown in Figure 6a). Expanding upon this concept, the extensive image database is administered through a Web Geographic Information System (Web GIS) platform. This platform enables the visualization of drone images, each containing embedded GPS coordinates, at their respective geographical locations on the map provided (as shown in Figure 6b) [23,24,25,26]. The image library currently contains approximately 6000 km of relatively continuous Ming Great Wall structures, which include Hollow Enemy Towers, Beacon Towers, and castles. By employing this platform, it becomes feasible to swiftly access high-definition images spanning the entire extent of the Ming Great Wall. This process aids in the initial recognition and pinpointing of Hollow Enemy Towers distinguished by white plaster remnants on their outer surfaces.

2.3. Drone Close-Range Photogrammetry and Point Cloud Pseudo-Color Analysis of Plastered Hollow Enemy Towers

Close-range photogrammetry of the Hollow Enemy Towers with exterior plaster is performed using the DJI Mavic 3E drone, filtering data from the “Comprehensive 3D Image Database of the Ming Great Wall”. This drone features a wide-angle camera equipped with a 4/3 CMOS sensor. The camera has a focal length of 12.29 mm, a pixel size of 3.3 μm, and an effective resolution of 20 megapixels. Using manually operated drones, close-range photogrammetric surveys were conducted on the four exterior facades of the Enemy Towers following a serpentine flight trajectory. This flight path ensures a minimum of 80% overlap ratio in both heading and side directions, creating a closed trajectory that covers the entire exterior of the tower. During the aerial survey, a consistent photographic distance of 1.5–2.5 m is maintained between the drone and the Hollow Enemy Tower (as shown in Figure 7a,b).

The high-resolution images acquired through close-range photogrammetry undergo processing using DJI Terra (2.3.3), a computerized 3D reconstruction software. This process includes aerial triangulation and 3D model reconstruction stages to produce a precise 3D model of the exterior facades of the target tower [27,28,29]. In recent years, researchers have begun to analyze 3D point cloud models through pseudo-color imagery to study issues such as material degradation on building surfaces. Li et al. utilized the pseudo-color analysis function of CloudCompare software (2.12.2) to visually identify cracks and uneven bulging on the walls of the Great Wall’s Enemy Tower (2018) [30]. Galantucci et al. introduced the 3D surface morphology analysis software Mountains (10.0) to identify and quantify issues such as material loss and cracks on building surfaces (2018) [31]. Fehér et al. assessed material loss and weathering of the stone surface of a medieval church over a five-month interval using the pseudo-color method in CloudCompare software (2022) [32]. This study employs pseudo-color analysis of point cloud models with the aim of assessing the relationship between the structural layers of plaster and exterior walls based on the variations in surface colors of the Enemy Tower’s point cloud model. This approach is intended to preliminarily eliminate cases where the missing materials on the surface of the Enemy Tower have been repaired through plastering in recent years. The model is subsequently imported into the point cloud analysis software CloudCompare. By following the sequence of steps outlined as “Tools-Projection-Export coordinate(s) to SF(s)”, a pseudo-color analysis of the plastered surfaces on the exterior walls is performed. The Color Scale ranges from Red > Yellow > Green > Blue. The purpose of this analysis is to visually represent the constructional stratigraphic correlation between the plaster layer and the brick surface of the exterior walls by employing various color thresholds that correspond to the variations in the point cloud along the y-axis (i.e., the vertical surface) [30,31,32,33].

2.4. Carbon-14 Dating of Plant Fibers in Plaster Mortar

Previous research has demonstrated that the application of radiocarbon dating to organic materials present in mortar can assist in establishing and enhancing the archaeological timeline of the examined structures [34]. A field survey was carried out, and mortar samples were collected from selected external plastered Enemy Towers. Through microscope observation, plant fibers appropriate for dating purposes were identified in 4 mortar samples, namely from Hollow Enemy Towers of Xuliukou No. 02 (MI1), Xuliukou No. 03 (MH1), Daomaoshan No. 03 (MG1), and Yumuling No. 06 (MC1). The mortar samples that were gathered underwent a gentle crushing and sieving process, during which plant fibers were meticulously sought out and retrieved from the pulverized samples (as shown in Figure 8 and Figure 9). The plant fibers samples were forwarded to the School of Archaeology and Museology at Peking University for Accelerator Mass Spectrometry (AMS) carbon-14 testing.

The plant fiber samples were preprocessed using the der Vries method, more precisely the Acid-Base-Acid (ABA) method [35]. This method entails a sequential treatment of the samples with hydrochloric acid, sodium hydroxide solution, and hydrochloric acid again to eliminate carbonates, humic and fulvic acids, and any atmospheric carbon dioxide that may have been absorbed during the procedure. The concentrations of solutions, treatment temperatures, and durations vary among laboratories based on the preservation state of the samples.

The specific preprocessing steps for the plant fiber samples utilizing the ABA method in this study are outlined as follows:

(1) The sample should be boiled in 2 mol/L hydrochloric acid, cooled naturally, and subsequently rinsed with deionized water until reaching a neutral pH level.

(2) The sample should be immersed in a 2% sodium hydroxide solution for 20–30 min, the duration depending on the sample’s condition. Subsequently, rinse the sample with deionized water until it reaches a neutral pH level, and the solution appears clear and colorless.

(3) The sample should be immersed in a 2 mol/L hydrochloric acid solution for 20 min, followed by rinsing with deionized water until reaching a neutral pH level.

(4) The sample should be transferred to aluminum foil and dried at 90 °C for a minimum of 12 h.

(5) The dried sample should be wrapped in aluminum foil, placed in a plastic sample bag, and the laboratory number should be recorded.

Before conducting radiocarbon dating, it is essential to transform pretreated plant fiber samples into graphite. The primary procedure consists of three stages: carbon dioxide generation, gas refinement and accumulation, and graphite formation. Carbon dioxide is produced by oxidizing organic samples like plant fibers and charcoal with copper oxide. Subsequently, the liquid nitrogen, characterized by its lowest boiling point, is employed to freeze carbon dioxide and water based on the principle of varying boiling and sublimation points of gases. The vacuum system subsequently removes additional impurity gases. Subsequently, an alcohol–liquid nitrogen mixture, adjusted to a boiling point that falls between the sublimation point of carbon dioxide and the freezing point of water, is employed to freeze the water, thereby enabling the collection of pure carbon dioxide gas. Iron powder serves as a catalyst in the hydrogen reduction process of carbon dioxide to facilitate the production of graphite. The chemical reaction is depicted as follows:

$${\mathrm{C}\mathrm{O}}_2+2{\mathrm{H}}_2\stackrel{\mathrm{F}\mathrm{e}\left(540 °\mathrm{C}\right)}{\to }\mathrm{C}+2{\mathrm{H}}_2\mathrm{O}$$

(1)

The water produced during the reaction is absorbed by magnesium perchlorate in the system, while the graphite is deposited on pre-cleaned aluminum foil using alcohol-soaked cotton. The sample is preserved under vacuum conditions and should be dated promptly. It is crucial to emphasize the necessity of controlling the sample amount during sample preparation to prevent fractionation effects. This control ensures that all carbon present in the sample is captured as carbon dioxide and subsequently transformed into graphite. The plant fiber sample usually has a weight ranging from 3.5–4.2 mg. If the quantity exceeds the reactor’s capacity, a portion of the gas needs to be vented, potentially leading to fractionation. Conversely, if the quantity is inadequate, it can result in a sample size that is insufficient, thereby amplifying the relative error in measurements conducted using accelerator mass spectrometry. The synthesis of graphite requires a reaction time of 6 h to ensure complete reduction of carbon dioxide to graphite [35].

2.5. Analysis of Plaster Mortar Materials Using XRD and XRF

During the field investigation of the exterior plaster of the Hollow Enemy Towers in Ji Town, samples were collected from various locations. These included the exterior plaster of Damaoshan No. 03 (MG1), Xuliukou No. 03 (MH1), Yumuling No. 02 (MF1), and Xuliukou No. 02 (MI1) Enemy Towers. Additionally, samples were taken from the binding mortar (MH2) found in the inner wall brick joints of Xuliukou No. 03 Enemy Tower. Additionally, mortar materials (ML3) employed in the restoration of the Hollow Enemy Towers at the Panjiakou section of the Ming Great Wall in Ji Town were also gathered (as shown in Figure 10). The mineral and chemical compositions of three types of mortar samples were subsequently analyzed comparatively.

X-ray Diffraction (XRD) analysis is employed for the identification of the mineral composition of plaster mortar samples [36,37,38]. Initially, surface contaminants are eliminated from the samples by employing a scalpel and a small brush. The samples are subsequently pulverized into a fine powder with an agate mortar until the texture of the powder is perceived as smooth and devoid of granules upon touch. The fine powder is subjected to drying in an oven at 45 °C until it attains a constant weight. The dried powder is subsequently subjected to analysis utilizing a Rigaku D/max 2500 X-ray diffractometer (Rigaku Corporation, Tokyo, Japan). XRD analysis captures diffraction patterns in a 2θ range from 10° to 90°, with a requirement for the spectrum collection duration to be over 20 min [39,40].

X-ray Fluorescence Spectroscopy (XRF) is utilized for the analysis of the chemical composition of plaster mortar samples. A Rigaku Supermini 200 XRF (Rigaku Corporation, Tokyo, Japan) is utilized for conducting semi-quantitative elemental analysis on the mortar sample powders.

2.6. Reconstruction of Historical Appearance of Ji Town’s Beacon-Equipped Hollow Enemy Tower

During the reconstruction of the Beacon-Equipped Hollow Enemy Towers in Ji Town to their historical appearance, images of the exterior plaster Enemy Towers in Ji Town were chosen from the comprehensive image database of the Ming Great Wall. The images were utilized for the creation of 3D models through the application of photogrammetric software. Utilizing orthographic images depicting the plastered surfaces of the towers, the locations and extent of the lime plaster were delineated and subjected to statistical analysis to ascertain the precise boundaries of the plaster on the waist walls of the towers. The reconstruction process also integrated references from historical documents, images, and physical remnants discovered within the existing structures of the Hollow Enemy Towers.

3. Results and Discussion

3.1. Density Analysis of Beacon Towers and Enemy Towers along the Entire Ming Great Wall

Figure 11 depicts a density analysis map illustrating the distribution of Beacon Tower locations along the entire Ming Great Wall. The analysis indicates that the regions with the most significant density of Beacon Towers are predominantly located in Xuanfu Town, Datong Town, and Chang Town. In contrast, Beacon Towers within the Ji Town defense sector exhibit a lower density along the length of the Great Wall compared to other areas. This discovery is contrary to expectations considering the significant strategic role of Ji Town as a crucial fortified region for the northeast of Ming Dynasty capital.

Figure 12 illustrates a density analysis map depicting the locations of Enemy Towers along the entire Ming Great Wall. In contrast to Figure 11, the most concentrated Enemy Tower structures are predominantly located in Ji Town and Chang Town, as well as in certain areas of Zhenbao Town, Datong Town, and Ningxia Town, with Ji and Chang Towns exhibiting the highest density of these towers. The substantial concentration of Enemy Towers highlights the military strategic significance of the segment of the Great Wall located in Ji Town. The results further support historical documentation, indicating that certain Hollow Enemy Towers in Ji Town had dual roles as Beacon Towers, consistent with empirical evidence.

3.2. Preliminary Screening of Hollow Enemy Towers with Plastered Exteriors along the Entire Ming Great Wall

Utilizing the “Comprehensive 3D Image Database of the Ming Great Wall”, a preliminary assessment was carried out to identify images of Hollow Enemy Towers along the entire stretch of the Great Wall displaying remnants of white plaster. The screening process revealed more than 50 Hollow Enemy Towers in the Ji Town area, characterized by remnants of white plaster on their exterior walls. Furthermore, comparable white plaster was discovered on the crenelated walls of specific Beacon Towers and their adjacent areas in Ji Town. If the identified exterior plasters are indeed original materials from the Ming Dynasty, this not only validates the historical accounts of Ji Town’s Beacon-Equipped Hollow Enemy Towers and Beacon Towers, but also contributes to and deepens the scholarly comprehension of the physical remains of Great Wall beacon structures and the historical landscape appearance of the Great Wall.

3.3. Drone Close-Range Photogrammetry and 3D Pseudo-Color Point Cloud Analysis

Figure 13a depicts a pseudo-color image showcasing the upper plastered area located on the east facade of Pingdingyu No. 13 Enemy Tower. The results indicate that the blue and green regions in the pseudo-color representation correspond to the weathered and eroded areas on the outer brick surface of the Enemy Tower. These changes are attributed to natural elements, resulting in recessed brick structures in comparison to the smoother sections of the external wall. In the pseudo-color image, the plastered sections of the brick wall exhibit a color variation in the mortar layer compared to the adjacent brick surface. The mortar layer, depicted in red, suggests its location on the outermost edge along the y-axis of the wall face, where it has been applied over the brick wall surface.

The pseudo-color image depicting the partial plastered wall surface of Zhuizishan No. 02 Enemy Tower reveals that the red areas lack an explicit correlation with the distribution of the plaster surface (as shown in Figure 13b). This observation suggests that the plaster was not applied over the brick wall surface. The color and texture of the mortar significantly differ from the mortar present at the brick joints. This pertains to the plastering and leveling procedures implemented to restore deteriorated and eroded brick wall surfaces in recent cultural heritage restoration projects. Therefore, relevant Enemy Tower instances are excluded.

3.4. Analysis of Carbon-14 Dating Results of Plant Fibers in Plaster Mortar

The carbon-14 dating results of the plant fibers contained in four mortar samples are presented in Figure 14. After applying dendrochronological correction [41], all samples have been dated to fall within the historical timeframe of the Ming Dynasty (1368–1644). The results presented in this study serve to conclusively demonstrate that the mortar samples collected correspond to the remnants of the outer wall plaster markings found on the Beacon-Equipped Hollow Enemy Towers of the Ming Great Wall in Ji Town, as documented in historical records.

Historical records highlight that upon Qi Jiguang’s appointment as the General of Ji Town, he presented a proposal in February of the third year of the Longqing era (1569). This proposal urged the central government to undertake the extensive construction of Hollow Enemy Towers in Ji Town and Chang Town [42]. The government granted approval for his request, leading to the construction of 472 towers within the same year. Subsequently, the Hollow Enemy Towers underwent multiple expansions until the ninth year of the Wanli era (1581). By this time, the construction of the Hollow Enemy Towers in both Ji and Chang Towns was predominantly finished, totaling 1448 towers. In the tenth year of the Wanli era (1582), Qi Jiguang was relocated from Ji Town to Guangdong [42]. The construction of the Hollow Enemy Towers in Ji Town and the practice of whitewashing the outer walls of the Beacon-Equipped Hollow Enemy Towers can be attributed to the efforts of Qi Jiguang during his leadership as the commander-in-chief in Ji Town (1569–1582). Therefore, it can be deduced that the timeframe in which the external walls of the Beacon-Equipped Hollow Enemy Towers in Ji Town were plastered also aligns with this period.

Among the carbon-14 dating results for plant fibers found in the four plaster samples, the calibrated calendar ages of samples MH1 and MC1 align with the historical period when the external walls of Ji Town’s Beacon-Equipped Hollow Enemy Towers were plastered (1569–1582). Specifically, the actual age of sample MH1 is situated in the earlier segment with a confidence level of 27.9% under a 2σ confidence interval. And the genuine age of sample MC1 is positioned in the later segment with a confidence level of 42.8% within a σ confidence interval (as shown in Figure 14).

The calibrated calendar ages of sample MI1 fall within the range of 1302 to 1405. This range predates the actual historical period (1569–1582) when the external walls of Ji Town’s Beacon-Equipped Hollow Enemy Towers were plastered by more than 160 years. The calibrated historical age of sample MG1 ranges from 1408 to 1444, predating the plastering period by at least 120 years. Nevertheless, the dating results of plant fiber samples MI1 and MG1 can still serve as strong evidence that the Xuliukou No. 02 and Da maoshan No. 03 Enemy Towers are indeed Beacon-Equipped Hollow Enemy Towers of Ming Dynasty. First, the dating results of the plant fiber samples MI1 and MG1 found within the mortar indicate a date corresponding to the Ming Dynasty, demonstrating that the plastering of the Enemy Tower’s exterior was not undertaken by later generations. The subsequent analysis of the mineral and chemical composition of mortar samples MI1 and MG1 also corroborates this point. More importantly, according to Qi Jiguang’s documentation in “Practical Record of Military Training”, the earlier Great Walls of Ji Town were low and mostly dilapidated, with small brick and stone towers constructed along the wall [19]. However, these towers were not built on the wall itself and lacked overhead protective structures, leaving soldiers exposed to the elements throughout the year. Prior to 1569, there is no evidence of the construction of Hollow Enemy Towers on the Ji Town Great Wall. Therefore, the actual date of the plastering on the exterior surfaces of the Xuliukou No. 02 and Damaoshan No. 03 Hollow Enemy Towers can only be after 1569.

As for the dating results of plant fiber samples MI1 and MG1, which are at least 120 to 160 years earlier than 1569, there may be two possible reasons for this discrepancy. Firstly, previous research has demonstrated that as carbon-14 dating approaches its upper limit of 1950, the margin of error tends to increase. Sun posits in her study that this approach is unsuitable for structures constructed after 1644 [43]. Given the proximity of the actual historical period of these 4 samples to this particular year, certain margin of error is to be anticipated. Additionally, the carbon-14 dating conducted in the laboratory determines the year of the plant fibers’ demise, rather than the year when the mortar was produced and utilized on the walls of the Enemy Tower. Therefore, a disparity exists between them. Qi highlighted in his work that the carbon-14 dating of wood components (or plant fibers) should not postdate the actual construction date if they were created concurrently with the architecture’s original construction [44]. The dating results of samples MI1 and MG1, which predate the actual historical period of exterior plastering of Ji Town’s Beacon-Equipped Hollow Enemy Towers (1569–1582), suggest the possibility that plant fibers that had perished years before were incorporated into the mortar during the plastering process of the corresponding Enemy Towers.

The carbon-14 dating results of the plant fibers extracted from the plaster on the exterior walls of the four Hollow Enemy Towers provide solid evidence confirming that the Xuliukou No. 02 (MI1), Xuliukou No. 03 (MH1), Damaoshan No. 03 (MG1), and Yumuling No. 06 (MC1) Enemy Towers are indeed valuable remnants of Hollow Enemy Towers with beacon functions from the mid to late Ming Dynasty. Based on this result, we infer that the remaining 49 Hollow Enemy Towers discovered along the Ming Ji Town Great Wall, which also have remnants of exterior wall plaster, should similarly be Hollow Enemy Towers that served as beacons during the Ming Dynasty. This has confirmed and mapped the full distribution of the Beacon-Equipped Hollow Enemy Towers, as determined by carbon-14 dating, and other plaster-marked Hollow Enemy Towers along the Ming Ji Town Great Wall (as shown in Figure 15). The results indicate the presence of multiple remnants of the plastered Hollow Enemy Towers along the route from Banchangyu to Damao Mountain Village in Qinhuangdao City, Hebei Province. Various sites, including Dashiyu in Zunhua City, Xifengkou in Qianxi County, Tangshan City, Yumuling, Xuliukou in Qian’an City, and Dongfeng Village in Lulong County, Qinhuangdao City, exhibit remnants of plaster-marked Hollow Enemy Towers. Furthermore, white plaster residues were discovered at the crenelated walls of solid Beacon Towers such as Naziyu No. 04 and 05, Qinhuangdao City (as shown in Figure 4a and Figure 15). This result supports historical documentation indicating that the crenelated walls of solid Beacon Towers in Ji Town were initially coated with whitewash [22].

It is worth noting that the majority of the remnants of the plaster-marked of Beacon-Equipped Hollow Enemy Tower are situated not in the well-known tourist areas of the Great Wall, such as Badaling or Jinshanling, but rather in the less developed place of the Great Wall. This indicates a deficiency in acknowledging the heritage significance of Beacon-Equipped Hollow Enemy Towers and their signage, resulting in frequent renovation projects in tourist spots that harm these valuable historical informations.

3.5. Mineral and Chemical Composition of Plaster Mortar Materials and Construction Practices

(1) XRD Analysis

Figure 16 presents XRD patterns of partial mortar samples obtained from the Hollow Enemy Towers in Ji Town. Four samples of plaster from the outer walls of Hollow Enemy Towers (designated as MG1, MH1, MF1, MI1) were analyzed, with MG1, MH1, and MI1 representing towers that have been identified as Beacon-Equipped Hollow Enemy Towers of the Ming Dynasty through chronometric analysis, primarily composed of calcite, quartz, and dolomite. The diffraction peaks for dolomite in sample MI1 exhibit higher intensity levels in comparison to the other three samples.

XRD analysis of the mortar sample extracted from the crevices in the brick walls of Ji Town Hollow Enemy Tower (MH2) indicates that its predominant mineral phases include calcite, quartz, and dolomite, alongside other minerals. Liu conducted XRD analysis on the mortar samples extracted from the Ming Great Wall section located in Yanqing, Beijing, as documented in his doctoral dissertation. The results revealed that the predominant mineral structure in the mortar consists of calcite-type calcium carbonate, with quartz following as a secondary component [45]. Zhang conducted an analysis of samples obtained from various historical sites, including the Nanjing Ming Dynasty city walls, Jiayuguan Ming Great Wall in Gansu Province, and Dongjiakou Ming fortress in Funing County, Hebei Province. XRD spectra indicated that a prominent peak around the 2θ angle of approximately 30 degrees corresponds to the characteristic peak of the calcite crystal structure. He also observed that limestone frequently includes minor quantities of dolomite and clay impurities [46]. The results align with XRD analysis results of the four exterior wall plaster samples (MG1, MH1, MF1, MI1) and the interior wall brick joint mortar sample (MH2) examined in this study. This observation also suggests that the mortar applied between the plaster on the exterior walls and the brick joints of the Enemy Towers shares a similar mineral composition, possibly derived from a common manufacturing procedure.

XRD analysis of the contemporary mortar sample (ML3) obtained from the Panjiakou Great Wall restoration site reveals the predominant mineral phases present, which comprise calcite, dolomite, quartz, wollastonite, and hedenbergite. In contrast to the mortar samples obtained from the Hollow Enemy Towers of the Ming Dynasty, the present sample demonstrates a significantly higher peak intensity for dolomite at approximately 32°, exceeding the peak intensity of calcite at approximately 30°. XRD analysis reveals a difference in mineral phases between the modern mortar sample (ML3) utilized at the Panjiakou Great Wall restoration site and the mortar employed in the exterior walls and brick joints of the Hollow Enemy Towers.

(2) XRF Analysis

Table 2. XRF Analysis Results of Hollow Enemy Towers Mortar Samples from Ming Ji Town Great Wall (Wt%).

	CaO	MgO	SiO2	Al2O3	Fe2O3	Na2O	K2O	TiO2	SO3	P2O5
MG1	67.4	20.6	6.37	1.78	1.26	0.144	0.527	——	1.38	0.088
MH1	55.5	34.4	3.25	0.516	0.687	0.426	0.282	——	4.58	0.107
MF1	52.5	28.0	14.4	1.91	1.89	0.129	0.418	——	0.270	0.189
MI1	48.8	38.0	7.35	0.788	0.734	0.674	0.460	——	2.61	0.131
MH2	66.5	27.5	3.67	0.608	0.633	0.192	0.189	——	0.533	0.083
ML3	55.1	12.4	21.1	5.26	3.50	0.475	0.852	0.316	0.722	0.088

shows XRF results for the mortar samples gathered. With the exception of ML3, the chemical compositions and the proportions of various components in the remaining mortar samples exhibit similarities. Furthermore, the magnesium oxide content found in the mortar samples extracted from the external plaster of the Hollow Enemy Towers and the brick joints of the interior walls surpasses 5%, categorizing them as magnesian lime. This classification is consistent with the results of previous studies on the mortar materials of the Ming Great Wall in Ji Town [47,48].

In Table 2, the magnesium oxide content in the mortar sample (ML3) from the Panjiakou Great Wall restoration site is significantly lower in comparison to the other samples. Conversely, its silicon dioxide, aluminium oxide, and iron(III) oxide contents are relatively higher, aligning with the composition typically found in modern cement. Confirmation provided by the construction personnel present at the Panjiakou Great Wall restoration project indicated the utilization of modern cement in the mortar for the restoration of the Hollow Enemy Towers at the site. XRF results reveal that the material composition of the exterior wall plaster and interior wall brick joint mortar of the Ji Town Beacon-Equipped Hollow Enemy Towers is consistent, and different from the contemporary mortar employed in the restoration initiatives of the Great Wall.

3.6. Plastering Practices for Exterior Walls of Beacon-Equipped Hollow Enemy Towers

During the site survey conducted at the Ji Town Beacon-Equipped Hollow Enemy Towers, it was observed that sections of the white plaster mortar had delaminated, revealing the underlying raw clay layer beneath the limewash coating. The raw clay layer was discovered to be present on the outer brick surfaces of the Enemy Towers. Furthermore, within the block mortar samples gathered on the site, a stratum of unprocessed clay was found attached to the underside of the white plaster layer, along with sizable plant fiber fragments interspersed within the clay (as shown in Figure 17a,b). According to “Chinese Ancient Architecture Construction Techniques” [49], the plastering method referred to as “mud bottom lime” entails the application of clay as a base coat on the brick walls of the towers, followed by a finish coat of lime painted over the raw clay to serve as the outermost surface layer. To improve the bonding strength, clay may be supplemented with aggregates such as wheat chaff. Additionally, the limewash applied to the surface layer typically contains plant fiber.

3.7. Historical Appearance Reconstruction of the Beacon-Equipped Hollow Enemy Tower of the Ming Great Wall in Ji Town

(1) Statistical Analysis of the Plastered Locations of Beacon-Equipped Hollow Enemy Towers in Ji Town

Statistical results derived from orthographic images of Beacon-Equipped Hollow Enemy Tower’s remains in Ji Town indicate the preservation of plaster on the crenelated walls, at the junctions of these crenelated walls with the main wall bodies, and on the surfaces of the brick-built wall bodies. Further examination of the limewash application on the tower structure reveals that the limewash is predominantly observed above the arched vaults of the arrow silts within the Enemy Tower (as shown in Figure 18a).

(2) Referential reconstruction of historical appearance of Beacon-Equipped Hollow Enemy Towers in Ji Town

Based on the statistical results regarding the distribution of plaster on the exterior walls of the Beacon-Equipped Hollow Enemy Tower, a reconstruction project was undertaken to recreate the historical appearance of the Beacon-Equipped Hollow Enemy Tower along the Ming Great Wall in Ji Town during the Longqing and Wanli eras (1567–1620). According to the regulations established by Qi Jiguang, the Hollow Enemy Towers with beacon functions were identified by the application of limewash on its crenelated walls and around the waist walls, which are above the arch vaults of the arrow silts on the four sides of the tower body (as shown in Figure 18a,b). In the endeavor to faithfully recreate the authentic historical look of the Beacon-Equipped Hollow Enemy Towers, the reconstruction process encompassed more than only marking the relevant sections of the outer walls with limewash, as depicted. It also entailed thorough research and reconstruction of crucial beacon components that have mostly disappeared from the existing structures, including the roofed structures on top of the towers named Pufang and flagpoles (as shown in Figure 18c).

According to the “Chronicles of Four Towns and Three Passes”, each Hollow Enemy Tower featured a “small-sized house built at the top for soldiers to stay and store equipment”, and each Beacon-Equipped Hollow Enemy Tower was equipped with a flagpole, measuring approximately 5.76 m in height [17]. Qi Jiguang’s “Imperial Ming Legal Records” mentions that “small square yellow flags were used on both Hollow Enemy Towers and Beacon Towers, with wooden poles always hanging” [22]. The “Map of the Ming Ji Town Great Wall” displayed at the National Museum of China depicts images of the Hollow Enemy Towers in Ji Town. This artifact is a valuable resource demonstrating that each top of the Hollow Enemy Tower was adorned with a Pufang structure. Additionally, the illustration shows a flagpole with a flag hanging from it at one corner of the tower’s top (as shown in Figure 3). After over four centuries since their erection, the authentic rooftop dwellings situated atop the Hollow Enemy Towers lining the Great Wall have predominantly deteriorated (with the ones at renowned locations such as Badaling primarily being replicas), and the wooden flagstaffs positioned at the corners of the towers have been absent for a significant period. Upon examination of the “Comprehensive 3D Image Database of the Ming Great Wall”, an observation revealed that the summits of Enemy Towers include No. 09 of Dongjiakou and No. 11 of Pingdingyu, located in Qinhuangdao, Hebei Province, exhibit relatively well-maintained primary components of the initial Pufang structures of Ming dynasty (as shown in Figure 18c). The structures are characterized by brick construction featuring archways that open towards both the interior and exterior of the wall. It is worth noting that the Pufang structures depicted in the “Map of the Ming Great Wall in Ji Town” exhibit hip-and-gable roofs, while the preserved houses at No. 09 of Dongjiakou and No. 11 of Pingdingyu showcase roofs in the gable-style. Furthermore, an examination of the “Comprehensive 3D Image Database of the Ming Great Wall” uncovered several flagpole stone foundations situated at the summits of certain Hollow Enemy Towers in Zhenbao Town. This town served as another significant military garrison during the Ming Dynasty (as shown in Figure 18c). The bases provided vertical support for the flagpoles. According to the “Chronicles of Four Towns and Three Passes”, subsequent to the erection of Hollow Enemy Towers in Ji Town under the supervision of Qi Jiguang, other military garrisons such as Zhenbao Town emulated and commenced widespread construction of towers within their administrative boundaries [17]. The identification of flagpole stone bases in Zhenbao Town aligns with both historical records and image evidence of flags being displayed atop Hollow Enemy Towers.

4. Conclusions

The study examines historical records, including Qi Jiguang’s writings, utilizing a multidisciplinary technical methodology that incorporates Geographic Information Science, Surveying and Mapping, Computer Science, Archaeology, Materials Science, History, and Architecture. The comprehensive methodology employed in this study facilitated the discovery and precise identification of the plaster markings on the Beacon-Equipped Hollow Enemy Towers of the Ming Great Wall in Ji Town, thereby establishing their distribution along the entire length of the wall. The research also investigated the mineralogical and chemical composition of the exterior wall plaster mortar, which serves as a crucial indicator of the Beacon-Equipped Hollow Enemy Towers in Ji Town, along with the construction methods utilized. The original historical appearance of these towers was meticulously reconstructed according to the research results. The primary findings of the research are outlined as follows:

• During the Longqing and Wanli periods in the Ming dynasty (1567–1620), the Hollow Enemy Towers of the Great Wall in Ji Town were constructed on a significant scale under the supervision of Qi Jiguang. These towers were not only built for garrison, storage, surveillance, and weapon firing purposes but also served dual functions as Beacon Towers. The structures, referred to as Beacon-Equipped Hollow Enemy Towers, were instrumental in the dissemination of early warning signals. They were commonly differentiated from standard Hollow Enemy Towers by their whitewashed appearance with lime on the crenelated walls and waist walls on each of the four sides. Furthermore, the pre-existing solid Beacon Towers were also identified by the application of whitewash on their crenelated walls.
• Through comprehensive screening and confirmation with archaeological chronology and analysis of mortar material composition, four Hollow Enemy Towers along the Ming Ji Town Great Wall have been identified as remnants of Beacon-Equipped Hollow Enemy Towers of Ming Dynasty. These are the Xuliukou No. 02 (MI1), Xuliukou No. 03 (MH1), Damaoshan No. 03 (MG1), and Yumuling No. 06 (MC1) Enemy Towers. Based on this, we infer that the remaining 49 Hollow Enemy Towers found along the Ming Ji Town Great Wall, which also have plaster remnants on their exterior walls, should similarly be remnants of Beacon-Equipped Hollow Enemy Towers of Ming Dynasty. These towers are situated in the contemporary Hebei Province, specifically in Dashiyu in Zunhua City; Xifengkou and Yumuling in Qianxi County, Tangshan City; Xuliukou in Qian’an City; Dongfeng Village in Lulong County; and from Banchangyu Village to Damaoshan Village in the Haigang District, Qinhuangdao City. The placement of all these towers falls under the purview of Qi Jiguang’s leadership as the General of Ji Town during the mid to late Ming Dynasty.
• XRD analysis of the exterior wall plaster samples from the Beacon-Equipped Hollow Enemy Towers reveals that the predominant phases present in the plaster mortar are calcite, quartz, and dolomite. Furthermore, the results of XRF suggest that the magnesium oxide content in the plaster samples surpasses 5%, thereby categorizing them as magnesian lime. This result aligns with the material analysis results of the mortar utilized in the interior wall brick joints of the towers and demonstrates difference from the contemporary mortar employed in Great Wall restoration endeavors. The inference can be made that the exterior wall plaster and the interior wall joint mortar of the Beacon-Equipped Hollow Enemy Towers were produced using the same source of materials and manufacturing processes.
• Field investigations have indicated that the Beacon-Equipped Hollow Enemy Towers’ exterior wall plaster markings of Ming Ji Town Great Wall were applied following the Chinese traditional construction practice of the Ming and Qing dynasties (1368–1912), known as “mud bottom lime”. This method entails the initial application of a mud mixture combined with straw and other aggregates as the foundational layer on specific sections of the Enemy Tower’s exterior walls. Subsequently, a white lime plaster is applied as the outermost layer, usually blended with hemp fiber and other plant fibers.
• Based on a statistical analysis of the preserved locations of the exterior wall plaster on the Beacon-Equipped Hollow Enemy Towers and historical descriptions of the whitewashed areas on these towers, the study determined the specific coverage area of the exterior wall plaster markings. The trapezoidal region is situated between the crenelated walls at the pinnacle of the tower and the arch vaults of the arrow silts. Furthermore, by utilizing pertinent historical images, documentary records, and physical artifacts, a reconstructive illustration depicting the historical appearance of the Beacon-Equipped Hollow Enemy Towers of Ming Ji Town Great Wall was created.

The study’s results enhance the typology and morphology of beacon architecture along the Great Wall, providing a significant supplement to the current understanding of the original appearance of the architectural landscape of the Great Wall. This research is significantly enlightening for the reenactment of historical scenes, as well as for the interpretation and presentation of the heritage values associated with the Great Wall. Field investigations have shown that the crucial identifying features of the Ji Town Beacon-Equipped Hollow Enemy Towers and Beacon Towers, namely the exterior wall plasters, are deteriorating rapidly and peeling off at a concerning pace. Furthermore, a significant portion of the remains of the Beacon-Equipped Hollow Enemy Towers are experiencing substantial structural problems, including weathering and material deterioration, which increase the likelihood of collapse. There is an immediate requirement for specialized protection projects aimed at rescuing the beacon architecture along the Great Wall and preserving their exterior wall plaster markings.