2 Samuel 5:7

The strong hold of Zion] called later on the 'city' (i.e. citadel) 'of David,' because he built and fortified it. It is to be noticed that the city of David does not mean the city of Jerusalem but the fort on Mt. Zion. David does not yet venture to live in Jerusalem itself, outside the fort.

The city of Jerusalem is built on high ground, which is shaped like a cloven tongue; and it is probable that the 'city of David' occupied the eastern 'tip,' behind which rose in later times the Temple. Gradually the city spread to the western 'tip' of the tongue. 'Millo' appears to be the name of the part of the city which was not fortified; its meaning is uncertain; later on it also was included within the fortifications. The original 'city of David' is thus a triangle, two sides of which are naturally protected, and the third, probably at this time, as later, artificially. Below the height on which the fort is built is the Kidron valley.