Jamieson, Fausset and Brown on Jeremiah 21:13

13. inhabitant of the valley, and rock of the plain--Jerusalem personified; situated for the most part on hills, with valleys at the bottom of them, as the valley of Hinnom, &c.; and beyond the valleys and mountains again, a position most fortified by nature, whence the inhabitants fancied themselves beyond the reach of enemies; but since God is "against" them, their position will avail nothing for them. The "valley" between Mount Zion and Moriah is called Tyropoeon. Robinson takes, "rock of the plain" as Mount Zion, on which is a level tract of some extent. It is appropriately here referred to, being the site of the royal residence of the "house of David," addressed (Jr 21:12).