LT: Commander, we are moving some vehicles to give our units cover. Commander: NO! Just throw a squad of 3 out there. Give them 1 airstrike evry now and then but that is all. Lt: But sir, the zombies sprint and all the infected in the city will charge them! Captain: I said no! I want this to look dramatic so I will look like a cool person.
I think there should be a hostile survivor colony who you battle with for controll over the city. That way when your guys jump over the walls do to morale, they go there to make it more interesting. And same thing if the enemy's morale is low.