John Gill Commentary 2 Samuel 12:26

"Now Joab fought against Rabbah of the children of Ammon, and took the royal city." — 2 Samuel 12:26 (ASV)

And Joab fought against Rabbah of the children of Ammon
Of his being sent against it, and of his besieging it, we read in (2 Samuel 11:1) ; but it can hardly be thought that he has been so long besieging it, as that David had two children by Bathsheba; but the account of the finishing of it is placed here, that the story concerning Bathsheba might lie together without any interruption:

and took the royal city ;
or that part of it in which the king's palace was, and which, as Abarbinel observes, was without the city, as the palaces of kings now usually are.