Dungeon Defender. Play as the Dungeon Master and send your minions to defeat the heroes!
Control the destiny of your avatar with wasd or the arrow keys. Spacebar pauses. You can hold down ctrl when you select to pick the building under the cursor no matter how crowded a tile is. Pressing escape or x clears current selection. Basic strategy is to build lairs which produce creatures to fight back the hordes. Your avatar will die if left alone so make sure you watch his hit points.