Introduction to the Yan Xishan fortification group: The Yan Xishan fortification group is located in the Tuoliangfengding Mountains. Most of the caves here are between 2.5 and 3.5 meters high, with an entrance of 1.5 meters and a depth of 5 to 10 meters. The cave body is tall and can be walked freely. There are holes in the caves. There are holes in the caves. They are all arched with neat stones layer by layer, usually 2-4 layers. There is no adhesive between the stones. It looks simple, beautiful and extremely strong.