It occurred to Luna, perhaps slower than it should have, that the trap they'd placed on the bridge had spent its use on a far smaller force than they had anticipated. The Mareitanians now knew they were waiting for them, and after having the way to Ponyville cut off, would probably be interested in looking north. Admittedly the north was still a poor choice of route, but now it had been made clear that going south wasn't exactly a bed of roses either. If Faust did go north, all of Luna's forces were in the south, and only the fliers could get there in time to slow their advance.