2 Samuel 12:26

And Joab fought against Rabbah The narrative returns to the point at which it was left in ch. 2 Samuel 11:1. But how long a time was occupied in the siege does not appear. It is possible that it lasted more than one year, and did not come to an end till after the birth of Bath-sheba's first child. But on the other hand it would be quite natural for the historian, having once commenced his account of Bath-sheba, to complete it before narrating the capture of Rabbah, so that this may have been effected within a year.

and took the royal city "The royal city" seems to be equivalent to "the city of waters" of 2 Samuel 12:27, that is, the lower city on the river, as distinguished from "the city" (2 Samuel 12:28), i.e. the citadel. The capture of this probably deprived the citadel of its water-supply, and so rendered it untenable for any length of time.