"Zombie Apocalypse is a multidirectional shoot 'em up. The player controls one of four characters through 55 levels set in seven different areas. The player must rescue survivors, and kill waves of zombies using a range of weapons and the environment. Use of environmental kills rewards the player with more points for their score. Every five kills awards the player with another score multiplier, which resets to zero upon death. Each of the game's modes can be played in single or multiplayer.
There are 12 trophies/achievements available. Playing through the game unlocks new modes."
Source: Wikipedia, "Zombie Apocalypse," available under the CC-BY-SA License.