2 Samuel 5:9

9So David lived in the stronghold and called it the City of David. And he built all around [the surrounding area] from the 5:9 The exact nature of the Millo is unclear, but most scholars believe it was some sort of military fortification such as a tower, a citadel, a significant part of a wall or even an earth-fill or terraced rampart. It was located on the eastern side of the City of David. It was later repaired by King Hezekiah.Millo [fortification] and inward.