Wubei Zhi (Vol. 47)

...defensive platform or watchtower.

At the battlements original: 垛口 (duo kou); the openings in the parapet through which soldiers defend the wall. atop each watchtower, three soldiers shall be stationed per battlement. Select those who are physically strong and courageous for this duty. They are to lean outward to keep watch, while keeping their backs toward the interior of the city. Whether looking near or far, they must remain vigilant and look out for one another.

Coordinating Defense Zones and Signals

Each watchtower shall be assigned a specific designation original: 字號 (zi hao); usually taken from the "Thousand Character Classic," acting as a numbering system like "Sector A, B, C." and divided into Designated Defense Zones original: 信地 (xin di); a specific sector of the wall where a unit is held accountable.. These zones are further distinguished by signal flags, gongs, drums, and other signaling devices original: 響器 (xiang qi); literally "sound instruments.". During the day, the soldiers must observe the specific flags; at night, they must listen for the designated sound signals.

Furthermore, the tower's designation, the specific flag name, and the type of signaling device must be written in large, bold characters upon the parapet walls original: 垛牆 (duo qiang) so that every soldier can recognize them clearly. The watchtower itself serves as the primary point of reference for all movements.

For example, if the "Heaven-series" original: 天字號 (tian zi hao); the first designation in the sequence. watchtower is on the left and the "Earth-series" original: 地字號 (di zi hao); the second designation. watchtower is on the right, the space between them is divided into sectors. The Heaven-series tower manages a specific number of battlements extending toward the Earth-series tower, and the Earth-series tower manages those extending back toward the Heaven-series tower. The boundary of their designated zones meets in the middle.

Suppose the "Heaven-series" tower is responsible for forty battlements. These are further divided into groups of ten, with ten battlements constituting one "number" (or section).

• Section One: For these ten battlements, a red flag and a drum are used. The following eight characters shall be written in large script upon the parapet wall:
  "Observe the Red Flag by Day, Listen for the Drum by Night."
• Section Two: For these ten battlements, use...