Adam Clarke on 1 Kings 9:15

This is the reason of the levy - That is, in order to pay Hiram the sixscore talents of gold which he had borrowed from him (Hiram not being willing to take the Galilean cities mentioned above; or, having taken them, soon restored them again) he was obliged to lay a tax upon the people; and that this was a grievous and oppressive tax we learn from 1Kgs 12:1-4, where the elders of Israel came to Rehoboam, complaining of their heavy state of taxation, and entreating that their yoke might be made lighter.

And Millo - This is supposed to have been a deep valley between Mount Sion and what was called the city of Jebus, which Solomon filled up, and it was built on, and became a sort of fortified place, and a place for public assemblies. - See Calmet.