Classic "very simple game that you can waste exactly 30 minutes on during your lunchbreak" sort of thing. Sort of just works like tower defence where you're trying to destroy a zombie stronghold while at the same time protecting a human one. Becomes a bit unbalanced once you realise you literally only need to spam shovel and shotgun guys to win.

Alternatively, you can become obsessive and try to speedrun through each of the stages as well, by overly meticulously understanding the games mechanics and planning accordingly.