The primary goal in the game is to stay alive for as long as possible, but there are several ways to go about this. You can choose to take a more offensive front and plant traps for zombies like exploding barrels, or a more defensive approach by building yourself a base complete with rocket launching turrets. With eight playing fields to choose from—four of them geared towards the offensive approach, and four towards the defensive—there is plenty of room to adapt your own style of fighting the zombie horde. However, be warned that choosing the levels with defenses already built for you will start you at a higher level in the game.
The upgrade system for your weapons is quite unique in that it is based on combos. The higher your combo the more things you unlock. It starts out pretty easy, getting a combo of 5 kills will score you a shiny new shotgun, but it takes a combo of 500 to get the nuclear strike. Your Kill combo is displayed at the top right of the screen and you will notice the numbers slowly fade; each time it fades your combo will decrease by one. The higher the number the faster it fades so make sure you are killing faster than it is decreasing! Lucky for you, your explosions don't hurt you, so don't be afraid to use yourself as bait to lure a bunch of zombies into a row of barrels and blow them all up.