It doesnt matter how many zombies are outside the window, as long as you hold down x and rebuild the window while there taking it down they will never get through. Dont forget that while your holding off the zombies at the window you can kill all the other zombies coming at you while you rebuild the window. I tend to do that with a window that is in a corner of the room so the zombies can never sneak up behind me! Good tactic if you are playing a SOLO match. Another good tactic while playing CO-OP zombie matches is to keep the last zombie outside the window and keep rebuilding it while your team rebuilds all other windows and uses there money at the random box. Its hard playing with a team that wont rebuild windows BEFORE the next round and when the next round comes they want to stay at the random box for a gun. I hate that!!!