Alma 53:5

And this city became an exceeding strong hold ever after; and in this city they did guard the prisoners of the Lamanites; yea, even within a wall, which they had caused them to build with their own hands. Now Moroni was compelled to cause the Lamanites to labor, because it were easy to guard them while at their labor; and he desired all his forces, when he should make an attack upon the Lamanites.

This city became a very strong place. They kept the Lamanite prisoners inside this city. They kept them inside the wall that the prisoners built with their own hands. Moroni made the Lamanites work because it was easy to guard them while they worked. He wanted all his soldiers ready for when he attacked the Lamanites.

From that day on, the city became an incredibly strong fortress. They kept the Lamanite prisoners there, locked inside a wall the prisoners had built with their own hands. Moroni made the Lamanites work because it was easier to guard them while they labored, and he wanted all his forces ready when it came time to attack.