Each planet has 10 revolutions, which get harder, but the rewards get bigger. In each revolution there are 50 waves. The total number of waves for each planet is 500.
The fourth planet, Bokor, has ten hordes. To advance to the next horde, players must kill all the zombies. As you advance through hordes, enemy attack and health is increased and enemies become more numerous.
Your level determines your max health. Leveling up is achieved by collecting experience through killing enemies, pick-ups, Bro-Op challenges, or by recieving gifts from your friends. Certain guns have certain level requirements noted by the "Requires Level:" section in the expanded desciption of the weapon. The maximum level possible is level 200.