Alma 53:4

and he caused that they should build a breastwork of timbers upon the inner bank of the ditch; and they cast up dirt out of the ditch against the breastwork of timbers; and thus they did cause the Lamanites to labor, until they had encircled the city of Bountiful round about with a strong wall of timbers and earth, to an exceeding height.

He made them build a wooden wall on the inside of the ditch. They threw dirt from the ditch against the wooden wall. This is how they made the Lamanites work. The prisoners built a strong wall of wood and dirt around the city of Bountiful. They made it very high.

He had them build a timber wall on the inner side of the trench, then pile the dirt from the ditch against the wall. In this way they forced the Lamanite prisoners to labor until they had surrounded the city with a towering wall of timber and earth.