Adam Clarke on 2 Samuel 12:26

And took the royal city - How can this be, when Joab sent to David to come to take the city, in consequence of which David did come and take that city? The explanation seems to be this: Rabbah was composed of a city and citadel; the former, in which was the king's residence, Joab had taken, and supposed he could soon render himself master of the latter, and therefore sends to David to come and take it, lest, he taking the whole, the city should be called after his name.