Cities in Rebuild are overrun by Zombies. They're hiding on almost every building, even if you can't see them. The only place that's safe is inside your fort, and that might not be safe for long if your buildings aren't well defended enough. The number of zombies on each building gradually increase every day. They're attracted to the activity inside your fort and are more likely to crowd around outside it. If enough zombies get together against one of your fort's walls, they will turn into a zombie unit and attack the fort shortly after.
You can see how many zombies are in a building by clicking on it, and kill them via a kill zombies mission.