Albert Barnes Commentary Jeremiah 51:12

"Set up a standard against the walls of Babylon, make the watch strong, set the watchmen, prepare the ambushes; for Jehovah hath both purposed and done that which he spake concerning the inhabitants of Babylon." — Jeremiah 51:12 (ASV)

Upon the walls of Babylon - Or, “against the walls.” The King James Version takes the word ironically, as a summons to Babylon to prepare for her defense; others take it as a summons to the army to make the attack.