Alma 50:4

and he caused towers to be erected that overlooked those works of pickets; and he caused places of security to be built upon those towers, that the stones and the arrows of the Lamanites could not hurt them.

He built towers that looked down over the fence. He made safe places on top of the towers so that the stones and arrows from the Lamanites could not hurt them.

They erected watchtowers that overlooked the picket fences, with protected platforms where Lamanite arrows and stones couldn’t reach them.