Albert Barnes Commentary 2 Chronicles 32:5

"And he took courage, and built up all the wall that was broken down, and raised [it] up to the towers, and the other wall without, and strengthened Millo [in] the city of David, and made weapons and shields in abundance." — 2 Chronicles 32:5 (ASV)

The breaches in the wall of Jerusalem were not entirely due to the old hostility of Joash; they may have been caused either by neglect and carelessness in the reign of Ahaz (2 Chronicles 28) or by the simple process of natural decay. For his repairs, Hezekiah pulled down houses (Isaiah 22:10).

Regarding Millo, see the notes in the marginal references.