Passes -Great Wall Of China

As major strongholds along Great Wall of China , passes usually located at such key positions as intersections with trade routes. The ramparts of many passes were faced with huge bricks and stones, with dirt and crushed stones as filler. The bastions measured some 30 feet (10 meters) high and 13 to 16 feet (4 to 5 meters) wide at the top. Within each pass were access ramps for horses and ladders for soldiers. The outside parapet was crenellated, and the inside parapet, or duo-qiang (nu-qiang), was a low wall about 3 feet (1 meter) high that prevented people and horses from falling off the top.

In addition to serving as an access point for merchants and other civilians, the gate within the pass was used as an exit for the garrison to counterattack raiders or to send out patrols. Under the gate arch there was typically a huge double door of wood. Bolts and locker rings were set in the inner panel of each door. On top of each gate there was a gate tower that served as a watchtower and command post. Usually it stood one to three stories (levels) high and was constructed either of wood or of bricks and wood.

Built outside the gate, where an enemy was most likely to attack, was a weng-cheng, a semicircular or polygonal parapet that shielded the gate from direct assault. Extending beyond the most strategic weng-chengs was an additional line of protection, the luo-cheng, which was often topped by a tower used to watch those beyond the wall and to direct troop movements in battles waged there. Around the gate entrance there was often a moat that was formed in the process of digging earth to build the fortifications.

You may interesting in the other kinds of designs for the fortifications on the wall:
Platforms ,Dirt Walls and Stone Walls , Beacon towers ,passes,Walls