This is not your typical tower defense game but it follows that style of game. The basic concept is that you have a map with designated spots where you build your “towers,” in this case your troops, machine gun turrets, and in some cases towers. However, where this vastly differs from other tower defense games is in two areas (there are more ways than these but these are the basics). The first is that you are not defending a base (unless you choose the TV Challenge). You are defending the lives of your units. If all of your troops get killed, you fail the mission. So you have to keep at least one of them alive against waves of zombies. The second is that you can move your troops around on the map. Is one of your positions getting overrun by zombies? No worries, direct your unit to move to a different spot on the map. Their health regenerates while they run and they can’t take damage while running. In other tower defense games, if you pick a spot to place your “tower” you are stuck in that spot unless you spend the money/resources to build a second tower. In Zombie Defense, just tell your unit to run to a different location (unless you built a stationary gun turret or tower). These two differences have kept the fun going for me as I have to pay attention to keeping my men from getting eaten. Usually I get tired of tower defense games before I ever reach the end. However, all the different upgrades, units, and different types of play have kept me enjoying this game for months now! Regarding the different types of play, there are 5 different game types: Regular, Nightmare (increased difficulty), 3-Man Challenge (only allowed to play with 3 units), TV Challenge (protect the TV Cameraman), and Boss Challenge (a Boss appears during each wave of zombies). I cannot say enough good things about this game! If you have not tried Zombie Defense, download it now!