Headcount
Ghouls are closing in on my head. It swings its sword at any that get close, but it can't win alone: each one knocked down leaves a grave, and raising brings every grave back up on your side (the strongest first, once your army's nearly full), to come to you and fight. Watch out for spitters, bloaters that burst, wraiths that slip past your army, and a lord every fifth wave. Move with WASD or the arrow keys (or hold the mouse where you want to go, or drag a finger). Raise with Space, a right-click, or the Raise button. Clear a wave for a heart back.