Instead of building towers and defending against hordes of monsters, play the opposite as an evil mage that builds his army from the ground up to dominate the lands.
You are the commander of a space battle station. Below you, on the planet's surface, enemies are trying to capture deadly missile silos. Don't let them seize these silos. Protect them at all costs!
Zombies and ghosts are invading! Defend the local neighborhoods by building an array of towers to shoot at them and open gates to redirect their hungry minds to places of choice.
