Ancient Chinese cities often circled by two huge walls. The inner wall is called " cheng", while the outer wall is called " guo". " Cheng Guo" is used to refer to the ancient city walls.
There are also moats surrounding the walls. In the past times, the moat was called " chi", and the inner city was called " geng". The city wall and the moat are called " cheng chi" as a whole. The ancient cities had an integrated architectural system, for example, there are 2 to 3 gates to the city. (Big cities usually have more gates. The Beijing inner city has 9 gates in the Ming and Qing Dynasties). There is always a tower over the city gate. The number of the tower is always the same as that of the gate. There are also four watchtowers at each corner of the city wall. In order to reinforce the city wall, a section of the city wall, also known as " ma mian" is projected to the outside in every certain distance. In the inner side of the city wall, cavalries with their horses can climb up the wall by the horse path.