Though the Great Wall of China can be trace back to The Spring and Autumn Period pre the Qin Dynasty, yet the Watchtowers on the Great Wall, call it Beacon Tower or Guard Tower if you prefer, had existed long before the Great Wall itself. Since the watchtowers had existed and taken a key role in watching over the enemy out at the furthest layer of the country, the earliest Great Walls been built up had linked those watchtowers as crucial sections for its defensive function. Construction continued during the ensuing Han dynasty and was completed during the Ming dynasty between 1368 and 1644.
Watchtower is the key part of the military construction. Very close to each other, brick towers in square are built atop the Wall. A famous general Qi Jiguang in Ming Dynasty had ever mentioned the specifications and structure of the watchtower in his On-the-Spot Record of Military Training. It describes the watchtowers are about 36 meters wide and10 to 12 meters high. They built the tower at a distance of every 10 or 100 footsteps at the crucial parts, and every 52, 140 or 200 footsteps at other parts. Neighboring watchtowers can support and reinforce each other during the war. The location of any watch tower has to be in a very strategic point such as atop of a turning on a mountain or a narrow valley along the great wall, which makes sure that the signals they were sending out can be well spotted in the first sight.
The watchtowers on the great wall are mainly 3 stories tall. The first floor can be used to station soldiers or store food and weapons. The second floor was serving for lookout. On the top of the towers, there are cannon which can fire at invaders. Once the invaders scaled the wall, soldiers could retreat inside the narrow towers, where enemies were forcing to enter in a single file, so that the defending soldiers could pick the enemies off one by one.
For better viewing points, watchtowers were generally built on high places like mountain peaks or hill tops. The high altitude ensured that the nearby towers and troops see the messages clearly. And it has to be ensured that any three watchtowers nearby could all be seen by each other for delivering message more efficient. The distance between beacon towers varied from 3 to 10 miles according to the local situation.
The main signals are fire and smoke plus with waving flags and others signals in some other rare occasions. According to the Ming dynasty rules, they would light smoke in the day time and light fires as signals in the night. Because this was very practical considering that smoke would be easier to be seen in the daytime and fire in the other hand was well spotted at night.
Fire and smoke signals were often used for sending massages tower by tower. And in order to tell exactly how much enemy were invading and how urgent the invade situation was, by the time of Ming Dynasty, the empery rulers had also formed a sophistication warning system carried by the smoke and fire signals sending via watch towers along the Great Wall.
Apart from military functions, the watch towers were also providing shelter and supply assistance for the merchants passing via the silk roads. The first and second floors were used as living quarters for the soldiers and storage rooms for grain and fuel. And there were enough supply stored there if the soldiers had to live on that and survive for a long time.
Besides, watchtowers were a good place to keep out of the wind and rain as the soldiers patrolled on the Great Wall and served automatically as general’s conducting centre if battle begins.
You may hear some Chinese call the signal fire “lang yan” 狼烟 which means “wolves’ smoke”. This is because in ancient times, armies sometimes used wolf dung as the burning material. It was said that the smoke from burning wolf dung would not disperse in the wind, which was good for transmitting messages immediately. In some other dynasties they were using straws or something else to make up a signal fire based on their local martial.
The watchtowers did not distribute in evenly distance. Some of them were very close to each other with a distance of less than half a mile; some of them were far away to each other with a distance of several miles.
The shapes of the towers were also various. Most of them were square, while some were round or rectangular. And the construction materials of them were different, either. The materials were depending on the local material supplies.