And this was the cause that he lifted up his hand against the king: Solomon built Millo, and repaired the breaches of the city of David his father. (1 Kings 11:27)
Solomon’s work on the Millo was apparently done after he had completed his work on the Temple and palace (see 1 Kings 9:15, 24). David had previously done much to strengthen this area in the ancient Jebusite city captured by him (2 Sam. 5:9; 1 Chron. 11:8).