...areas near lush water and grasslands. These are the routes that the enemy must pass through. Such locations easily incite their greed original: "xian kou" (涎口), literally "drooling at the mouth," a metaphor for provoking the enemy's desire to capture a resource-rich spot.. The enemy's plans to scout and spy on such places will be relentless. Furthermore, when they first arrive, they concentrate their full striking power into a single sudden thrust. When they are exhausted and retreating, they often vent all their malice and frustration upon us. Beside long rivers and vast waters, the lush grasslands are where the enemy will inevitably establish their camps. Once they have camped, their strength remains whole, and they will conduct their attacks in rotating shifts original: "fan dai" (番代), a tactic where fresh troops replace tired ones to maintain a continuous assault without rest for the defenders.. If the people within the fort must withstand heavy formations by day and engage in fire-fights by night, how can they possibly endure for long?

Fourth: Avoid Sand Accumulations. "Accumulation" original: "yong" (壅), referring to the clogging or burying of defenses by environmental factors. refers to land where the wind carries shifting sands. This is a common occurrence in the northern borderlands. When shifting sand passes over a trench, the trench is buried; when it passes a wall, the sand piles up against it. Now, a fort is built to protect its residents, and it relies entirely on its walls and trenches. If the walls are buried, they become a level path; if sand accumulates, the enemy can use the mounds to scale the defenses. In such places, earthen walls are not enough to maintain a defense.

Second Rule: The Strategic Positioning of the Fort