What does 2 Samuel 12:31 mean?

He forced the people of the city to work with saws, iron picks, and axes, and made them labor at brickmaking. He did this to all the Ammonite cities. Then David and his army went back to Jerusalem. 2 Samuel 12:31 - Modern Text Bible

(David puts the conquered people to hard labor, asserting control over the region. Afterward, he returns home with his army.)