Introduction to ancient tunnels: The 10,000-meter military tunnel in Zhangbi Ancient Castle is the essence of the scenic spot. The tunnel has a three-dimensional network structure and is divided into three layers: upper, middle and lower. The lowest layer is more than 30 meters above the ground. There are command kilns, general kilns, storage kilns, ambush kilns, mangers, traps, sky eyes, water wells, drainage channels, etc. in the tunnel, which extend in all directions and are integrated with the above-ground Mingbao. It is known as the "Underground Great Wall".