Upon the walls of Babylon; rather, toward the walls (as Jeremiah 4:6). The "standard" was carried before the army, to show the direction of the march. Make the watch strong. Not merely for the safety of the invaders, but to blockade the city.
Comp. the phrase, "Watchers [a synonymous Hebrew word is used] came from a far country" (Jeremiah 4:16); i.e. besiegers. Prepare the ambushes. To press into the city when the besieged have made a sally (as Joshua 8:14-19; 20:33, 20:37).