no offense but it is pretty lame at this stage, to slow, cannon suck, add some units, like 3 different, 1 fighter 1 bomber and 1 cruiser, the bomber is the only who can attak the planet while the cruiser produces attack dronse
I agreed with Kaapola except that it SSHOULD be a timelimit but you will gain more time by rescueing a survivor than you do now, and killing a zombie will give an extra second.
Also for cheese add some cops shooting zombies.