The Yumen Pass, or Jade Gate, a stop along the Silk Road, is located 90 kilometers northwest of Dunhuang. Emperor Wu of the Han Dynasty set up four prefectures and two defense posts—Yangguan Pass and Yumen Pass in the Hexi Corridor—in the Gansu territory in northwest China, which formed a complete fire-based warning system in the region.

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A significant component of China’s border defense in ancient times, beacon towers were used to deliver emergency signals via smoke in the daytime and fire at night. The system took shape during the Western Zhou Dynasty (1046-771 BC) at the latest.

According to historical records, King You of Zhou, the last king of the Western Zhou Dynasty, tried to amuse his favorite concubine Baosi by lighting warning beacons and fooling his vassals into thinking that the enemies were about to attack. The vassals arrived outside the capital only to find themselves laughed at by Baosi. Even after King You had impressed Baosi, he continued to abuse his use of warning beacons and lost the trust of the vassals.

After defeating the nomadic Xiongnu, who often raided the north, Emperor Wu of Han (156-87 BC) set up four prefectures—Wuwei, Zhangye, Jiuquan and Dunhuang—as well as two defense posts—Yangguan Pass and Yumen Pass in the Hexi Corridor—in the Gansu territory in northwest China, which have become important landmarks on the Silk Road.

Also, Emperor Wu extended the Great Wall in the region and established a complete fire-based warning system. These defense facilities played a significant role in preventing the invasion from the Xiongnu and Qiang peoples during the Western and Eastern Han dynasties (206 BC-AD 220).

At the beginning of the 20th century, Aurel Stein, a Hungarian-British archaeologist explored beacon towers of the Han Dynasty in Dunhuang. There was increasing research on the relics of ancient Chinese watchtowers after the discovery of Han bamboo slips in Dunhuang and Juyan in today’s Inner Mongolia.

 Chinese archaeologists and historians, including Wang Guowei, Lao Gan, Chen Mengjia, He Changqun and Wu Rengxiang, researched Han watchtowers based on excavated relics and unearthed Han bamboo slips. They focused on the types of beacon fires, the frontier defense system, the fire-based warning system and the location of towers. These studies offer a window into the frontier defense system and military life during the Han Dynasty.

 
Historical roles
Han bamboo slips recorded explicit directives on the beacon fire-based warning system. Generally, there were five varieties of watchtower signals during the Han Dynasty. In the daytime, straw or wooden cages covered by cloth and silk, flags, and pillars of smoke were used as warning signals.

Torches made of reeds were used mainly at night. Huge haystacks were used both in the daytime to make smoke and at night to set fires. Moreover, each variety was classified into subvarieties to perform different functions.

In addition, there was a corresponding reconnaissance system at the time. A long, soft sand belt was set up outside the fortress walls. The belt was smooth and traceless, so it was easy to know how many people or horses had passed and where they were headed. Stakes and ropes were positioned in the belt, watchtowers or other places for interception and warning.

For each beacon tower, soldiers were appointed to keep watch, monitor the enemy and use beacon fires to alert others according to situation they detected. Upon seeing the beacon fire, the next beacon tower followed suit. In this way, warning information was passed on successively along certain routes. Li Zhengyu, a research fellow from Dunhuang Academy, said there were seven warning routes in Dunhuang.

Also, scholars studied the shape and structure of beacon towers. Generally, a beacon tower consisted of a square mound, suites of rooms, lavatories, livestock stalls and a fortified courtyard within the small castle. With a width of 5 to 8 meters and a height of several meters, the mound had a flat top on which a small room, namely the watch tower, was built. The watch tower was surrounded by lower walls without spy holes.

Stairs were built or a rope ladder was used to go up and down the mound. The two sides of the mound were connected with the walls of the castle. Within the castle, there were suites of rooms. Some had beds made of earth while some had cooking stoves. These places were where garrison soldiers lived.

 
Significance of preservation
The vast Hexi Corridor was sparsely populated and rich in saline-alkali soil. As a result, many beacon towers, especially those in Dunhuang, are preserved. According to the third national archaeological survey, which was completed in 2011, 76 beacon towers were found in Dunhuang, accounting for nearly half of the total sites there. These ancient watchtowers once served as sentinels on the Silk Road and bore witness to its rise and decline.

However, it should be noted that beacon towers have been deteriorating. In fact, Stein recorded the locations of more than 100 beacon towers in Dunhuang during his exploration. In addition to natural weathering, man-made sabotage also led to their destruction.

This was because the public has little knowledge about and was unaware of the significance of the relics of beacon towers. As mentioned above, the academic circle has conducted comprehensive studies and made great achievements in this regard. However, these research results are not open to the public.

In 2014, “Silk Road: Route Network of Chang’an-Tianshan Corridor,” spanning from China to Kazakhstan and Kyrgyzstan, was added to UNESCO’s World Heritage List. This highlighted the importance of its protector and witness—beacon towers, making it more urgent to popularize related knowledge. Now, in the era of information, it is important to consider how to translate written research results into images that are more accessible to the public.

 
Virtual reality
In recent years, the integration of science, technology and culture has become a new trend that has been widely applied in archaeology. In particular, three-dimensional visualization technology has been employed in such areas as archaeological survey and excavation, database construction, relics preservation and facial restoration of ancient people.

Through technology, archaeologists can record objectively and comprehensively all information obtained during explorations, which will provide data for further research and lay a foundation to reproduce the scene. Moreover, the imaging of the protected zone will facilitate planning for conservation of major sites.

With the aid of sensors and computer technology, VR uses headsets to generate realistic images, sounds and other sensations that replicate a real environment or create an imaginary setting. It also simulates a user’s physical presence in this environment. The technology has been used in exhibitions of cultural relics at home and abroad.

Egypt has adopted panoramic and 3D virtual technologies to establish an online experience center for the Pyramids. IBM is attempting to realize a virtual display of Beijing’s Palace Museum through VR technology. Some other museums have reproduced the scene of utensils being unearthed by means of VR.

By taking advantage of the technology, scholars from the Chinese Academy of Social Sciences are also trying to simulate historical sites, such as cave temples and ancient tombs. Such digitized displays make elusive archaeological concepts and knowledge more understandable to the public, which will contribute to the effective spread of academic achievements.

Previously, it was difficult to display the watchtower sites because most of the numerous beacon towers were located in the remote Gobi Desert. 3D visualization provides a way to reproduce each watchtower site, which has enabled the public to see beacon towers and their locations without being there in person.

In addition, VR technology helps create a dynamic beacon fire warning system. In human-computer interaction, one can experience the process of reconnoitering the sand belt, climbing the watchtower and setting the beacon fire. This offers a more vivid and effective way of communication and brings to life the research results on Chinese beacon towers.

 
Zhai Shaodong is from the Institute of Archaeology at the Chinese Academy of Social Sciences.