In the first great walls, wood was used to build the lookout tower. Later, with the arrival of bricks, the wood works were established on the spot at the time as the ovens. They were mainly used as a supply in beams, but also to fix the different tools. The buildings with a typical architecture with wooden beams date back from the end of the Ming time. Beforehand, the keystone system was prevalent. The gates were usually built with wood only.