Super Energy Apocalypse Recycled
build a base and keep it defended against a nightly zombie onslaught. The economy of building revolves not around money, but energy, of which there are many different sources, from coal to wind to nuclear. Each building you erect produces a certain amount of waste, which must be taken care of before it pollutes your base, because pollution to these zombies is like spinach to Popeye, and they grow mean and nasty real quick.
As Always, Good Luck and Have Fun.