Albert Barnes Commentary 2 Samuel 5:9

"And David dwelt in the stronghold, and called it the city of David. And David built round about from Millo and inward." — 2 Samuel 5:9 (ASV)

David dwelt in the fort - This refers to the stronghold (as in 2 Samuel 5:7), where he lived once the buildings were completed, which may have taken two or three years. Millo appears to have been a fortress of some kind, the northern defense of the city of David. It was likely part of the original Canaanite defenses of Zion, which seems probable because a fortress called the “house of Millo” also existed in the Canaanite city of Shechem (see the note on Judges 9:6 and 2 Samuel 9:1-20). Millo may be the native name. Some identify it with the great platform called the Haram es-Sherif.

David built round about - This probably means he built his own house and other houses and streets—in short, everything that caused it to be called the city of David (Compare 1 Chronicles 11:8). The buildings were located inside, on the south of Millo, so they were protected by it on the north, just as they were protected on the east, west, and south by the steep ravines.