Backs to the Wall is a Driving Event. It forces the party into an hour-long siege in a small, outdoor area. There will be multiple inaccessible buildings and lots of rubble, making it hard to maneuver around the hordes of zombies.
The group runs into a horde too thick to drive past, and gets cornered. They run for a good place to fight a siege, but the nearby buildings all look ruined. They are in a bad spot to fight.
SIEGE ALERT! THERE'S NO ESCAPE!