Took the royal city That is, that part of the city where the king's palace was; though now, it seems, he was retired to a strong fort. It is not to be supposed that Joab had continued the siege so long as till David had two children by Bath-sheba; this was done soon after the death of Uriah, when David commanded them to assault the city with greater force. The city of waters That part of the city which lay open, or was encompassed with the water; the other part, which was the upper city, and probably much stronger, was not yet taken.
Related Commentaries of 2 Samuel 12:26
2 Samuel 12:26-27
26 And Joab fought against Rabbah of the children of Ammon, and took the royal city.
27 And Joab sent messengers to David, and said, I have fought against Rabbah, and have taken the city of waters.