Jamieson, Fausset and Brown on 2 Samuel 12:27

27. the city of waters--Rabbah, like Aroer, was divided into two parts--one the lower town, insulated by the winding course of the Jabbok, which flowed almost round it, and the upper and stronger town, called the royal city. "The first was taken by Joab, but the honor of capturing so strongly a fortified place as the other was an honor reserved for the king himself."