Millo: (Heb. always with the article, "the" Millo).
Probably the Canaanite name of some fortification, consisting of walls filled in with earth and stones, which protected Jerusalem on the north as its outermost defence. It is always rendered Akra i.e., "the citadel", in the LXX. It was already existing when David conquered Jerusalem (2 Samuel 5:9). He extended it to the right and left, thus completing the defence of the city. It was rebuilt by Solomon (1 Kings 9:15,24; 11:27) and repaired by Hezekiah (2 Chronicles 32:5).
