Very cool.
A very neat little game. Nice progression. Just enough skill-based to make the randomness less damning.
The zombie soldiers towards the end with the bullet-proof vests and helmets was a nice touch. I was almost shouting at my little guys, "The legs!! Shoot the legs!!!" :)
But I didn't quite expect the vests to stop the chainsaw so effectively. Something you may want to address in a sequel. Or maybe not, who knows.
Some other things to consider for a sequal would be customizable survivors. I would have loved to used the hunting rifle in more levels but the number of runners made it impossible. But being able to give someone the Uzi and another person the AK would have made it doable.
Hell you might even go so far as to give your survivors statistics; names, accuracy, awareness, stamina, favorite weapon..... could be fun. :)