Rashi: There were two walls. The outer wall surrounded the capitol; the inner wall surrounded a fortress.

Radak: The city in which the king dwelled was called Ir ha'Mayim, because there was [a spring of] water in it. The city in which the people dwelled did not have water. Ir ha'Mayim was next to the other city, not inside it. Yo'av first conquered Ir ha'Mayim, and then the other city was easy to conquer, but he wanted David to conquer it, for David's honor.

Malbim: The king's palace was in a special fortress outside the city, like Mahari wrote. In it was the source of the water that the city used; therefore, it was called also Ir ha'Mayim. Yo'av knew that he will conquer it, for they will need water (and he conquered the city with the source of water).