Charles Ellicott 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)

The fort. —The same word as strong hold in 2 Samuel 5:7.

Millo. —A word always used in Hebrew with the definite article (Judges 9:20), the Millo. It is probably an old Canaanitish name for the fortification on the northern end of Mount Zion, “inward” from which the palace was situated. Subsequent kings, such as Solomon (1 Kings 11:27) and Hezekiah (2 Chronicles 32:5), saw its importance and added to its strength. On all other sides Zion was protected by precipitous ravines. There is, however, some difference of opinion about the topography of ancient Jerusalem.