Jehovah, standing, as it were, on the Mount of Olives, addresses the proud city beneath him. O inhabitant of the valley, and rock of the plain; rather, O inhabitress; Jerusalem is personified as a virgin.
The poetical description of the capital as a "valley" (the word, however, signifies a valley as wide as a plain) reminds us of "the valley [or rather, 'ravine'] of vision" (Isaiah 22:1, Isaiah 22:5); While "the rock of the plain" recalls "my mountain in the field" (Jeremiah 17:3).
So, as Graf points out, Babylon is called "a mountain" in metaphorical language (Jeremiah 51:25). It is, however, singular that the prophet should call Jerusalem a "valley" and a "rock" in the same passage.
In the former, perhaps, Jeremiah is thinking specially of the lower city, and in the latter of Mount Zion. Who shall come down against us? viz. from the "hills round about Jerusalem."