moat, ditch
塹 centers on a defensive trench or ditch dug into the earth, especially a moat surrounding a fortification.
塹 combines 斬 (cut, behead) with 土 (earth), suggesting cutting into the earth to form a ditch or moat.
The top part 斬 means to cut, and the bottom 土 is earth. Picture cutting a deep trench into the ground to create a protective moat.
For ザン, imagine a moat filled with the remains (残, ざん) of a defeated army: 残 -> ザン, and the ditch holds the aftermath.