The basic idea is that you have a limited number of ammunition and you must pick them back up. In addition to that the weapon will jam occasionally and requires un-jamming to continue firing. While the player needs to navigate around on the map to pickup their ammo, the enemies phase through walls and move directly towards the player.

The enemies will be upgraded as time goes on, eventually shooting their own projectiles back at you and at the hardest level they will shoot bouncy projectiles that will linger for 5 seconds or so. This is a basic "who can survive the longest".