Albert Barnes Commentary 2 Samuel 12:27

"And Joab sent messengers to David, and said, I have fought against Rabbah; yea, I have taken the city of waters." — 2 Samuel 12:27 (ASV)

The city of waters - This refers to the lower town of Rabbah (modern-day Amman), so named because of a stream that rises within it and flows through it. The upper town, which included the citadel, was situated on a hill north of the stream and was likely not defensible for long once its water supply was cut off.