Wubei Zhi (Vol. 48)

Cha Ban Insertion Board: Functions as a secondary door for the city gate. Its construction uses elm or pagoda wood, with width and height matched to the city gate. It is covered with raw cowhide and wrapped with iron plates. Iron rings are attached on both sides to thread through iron cables. For a large city gate, place two cheek timbers five feet away from the gate leaves. Open slots in the timbers, also wrapped in iron plates. When the enemy arrives, use a windlass (Jiao Che) on the gate tower to pull the threaded iron cables and lower the Insertion Board into the slots. The outside is reinforced to prevent fire attacks, and the inside is propped with timbers to prevent it from tilting or breaking.

Another method is to avoid the Insertion Board and instead drill dozens of holes in the gate. When the enemy approaches the gate, push out spears and halberds and shoot with heavy crossbows. This is called "Drilling a Secret Gate." In a location convenient for troops to sortie, secretly drill the city wall to create a gate, leaving about one foot of thickness without piercing it, to prepare for sorties to surprise the enemy. Its construction is seven feet high and six feet wide, with Pai Sha Zhu sand-shielding pillars installed inside and horizontal timbers for support.