The house of Millo - Was a royal palace, built by David; (see 2 Samuel 5:9;); and Silla is supposed to be the name of the road or causeway that led to it. Millo was situated between the old city of Jerusalem, and the city of David.
A conspiracy - Compare the marginal reference Joash, either from a suspicion of intended treason, or from some other unknown cause, took up his abode in the fortress of Millo 1 Kings 9:24. This conspiracy was connected with religion. Soon after the death of Jehoiada, Joash had apostatised; had renewed the worship of Baal; and, despite of many prophetic warnings, had persisted in his evil courses, even commanding Zechariah to be slain when he rebuked them 2 Chronicles 24:18-27. The conspirators, who wished to avenge Zechariah, no doubt wished also to put down the Baal worship. In this it appears that they succeeded. For, though Amaziah punished the actual murderers after a while 2 Kings 14:5, yet he appears not to have been a Baal-worshipper. The only idolatries laid to his charge are the maintenance of the high places 2 Kings 14:4, and a worship of the gods of Edom 2 Chronicles 25:14-20.
Silla - This place is quite unknown.