Adam Clarke on Jeremiah 21:13

O inhabitant of the valley, and rock of the plain - Dr. Blayney translates: "O thou inhabitant of the levelled hollow of a rock." With all his explanation I cannot see the good sense of this translation. Jerusalem itself, though partly on two hills, was also extended in the valley; and Zion, the city of David, was properly a rock, strongly fortified both by nature and art; and by its ancient possessors, the Jebusites, was deemed impregnable.

Who shall come down against us? - Probably the words of those courtiers who had persuaded Zedekiah to rebel against the king of Babylon.