Norman stands at the left of a graveyard and things come from the right. You cast, they fall, and what falls can be raised to fight alongside you, which is the loop: a wave that nearly killed you becomes the army that survives the next one. Clear enough and the game stops to offer an upgrade.
The upgrades are where the runs separate. They are small, specific and often strange, and two or three of them stacking in the right direction turns a careful defence into something that clears a wave before it arrives. Runs are short and death is quick. The pixel art is beautiful and the whole thing fits in thirteen kilobytes, which you would never guess from looking at it.