John Wesley on Jeremiah 21:13

Of the valley - The inhabitants of the city of Jerusalem are here intended. Jerusalem itself was built in part upon the rocky mountain of Zion, but a great part was in the valley, and the higher mountains, about mount Zion, made that mountain itself, in comparison with them, as a valley. The plain - Though also a rock of the plain, that is, near to the plain. Which situation of this place, made the Jews think it to be impregnable.