Wubei Zhi (Vol. 48)

Xian Ma Keng Horse-trapping pits: These are five feet long, three feet wide, and four feet deep. Inside the pit, plant Lu Jiao Qiang deer-horn spears and Zhu Qian bamboo spikes. Both items must be sharpened and hardened by fire. Arrange the pits in a cross pattern, or alternate the spears. Cover them with straw or grow grass seedlings on top to ensure the enemy does not notice them. These should be set up on all paths where the enemy might approach, both inside and outside the city gates.