The concept is that you are a tombstone (so you're constrained and things are out of your control). You are supposed to get out of the graveyard to reach the city gate before the other tombs and zombies destroy you.
The things that you can't control is the direction of the gun it shoots in a parabola. So you often miss the shots making it difficult to kill the zombies. Colliding with zombies also makes you lose the health. If you touch other tombs along the way, they start duplicating and you lose health.