The prisoners continued with their forced labor, and part of those labors was to reinforce the city of Bountiful. The Lamanites had nearly reached the city, and it was an important defense against any enemies that might come from the north. Although we have not yet heard of any specifically, Moroni nevertheless wanted that critical point strengthened, and used captive Lamanites to do the work.
Eventually, these prisoners are housed in Bountiful, and Moroni’s armies were freed to cover other locations.