Those square towers come out of the Great Wall on the enemy side. To facilitate the approach of the horses and four-wheel chariots, slopes were built along the Great Wall, near the towers. The three walls facing the enemies have all got three loopholes each, later called holes (Keng) or eyes (Yan) by the Chinese. Inside, there was a main arched room, where the warriors could oversee the four cardinal points, and a corridor surrounding this room. The light is provided by the loopholes and the entrance, looking out over the Great Wall. The inside stairs leading to the different levels take their bearing on the top of the wall (ground plan) As we can see on the sectional drawing, a small shelter was often built on the roof, with the doors and windows necessary to observe the enemy. The walls of the Shanxi regions are oblique and approximately twelve meters wide. Most of these towers were only accessible with ladders, in order to prevent the enemy from taking possession.

   In the region of Beijing, the towers are finer than in the region of Shanxi, and the walls are almost straight. Those defence towers cut the Great Wall, which gives them a better visibility on the rampart walk, but a weaker capacity for the tir de flanquement (propped up by rooms A&B, ridged out with loopholes and crenels). On the sectional drawing, we note that the Great Wall is formed of two thin brick walls. The gap between them is filled up with rubble-stone (rubbish and small stones) up to the breastwork. This space called “filler block”, allows a good draining and immobilises the bricks. The maximum thickness of the brick walls (0.5m) can be explained by the basic equipment of the wanderers, who had got neither onagers nor trebuchets. The building materials are the same for the two different types of towers: freestones for foundations up to two meters height; bricks for the walls, and terra-cotta flagstones for the rampart walk. To facilitate the building and the carriage, the doors’ and windows’ arch frame and threshold were pre-made before being carried and put together on the premises.  + photos ...