Albert Barnes Commentary Jeremiah 21:4

"Thus saith Jehovah, the God of Israel, Behold, I will turn back the weapons of war that are in your hands, wherewith ye fight against the king of Babylon, and against the Chaldeans that besiege you, without the walls; and I will gather them into the midst of this city." — Jeremiah 21:4 (ASV)

Without the walls — These words are to be joined to 'with which you fight.'