John Gill Commentary 2 Samuel 5:7

"Nevertheless David took the stronghold of Zion; the same is the city of David." — 2 Samuel 5:7 (ASV)

Nevertheless, David took the strong hold of Zion
A fortress without the city, and separate from it, and which was very strong; and the taking it might facilitate the taking of the city, which yet as appears by what follows, was very difficult to do:

the same [is] the city of David ;
it was afterwards so called, where he built a house, and dwelt.