The game was too easy. Zombies having only one action is unfair for the zombies. Humans can just stay two squares away and be safe. The game is playable, but not much more.
Give the zombies two action points or one move & one attack per turn if they shouldn't be able to attack twice.
Add more gameplay elements such as abilities, more weapons or stat customization.
Though if you're going to try to improve the game, I'd say just make a new game completely instead of updating this game. You could make Zombie Defense Force 2