I had an idea last night thinking about your game. You might consider adding Dungeon Wall pieces to advance with the zombies (to simulate walking through the corridor filled with monsters).
The wall pieces could look like this:
_ |_ _| |¯ ¯| ¯
I would recommend leaving the walls always open (no boxes) to avoid having a zombie trapped within it, or if you can ensure no zombie will be encased in an impenetrable wall, then add |¯| and |_| wall segments.
I hope that made sense.
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