Sorry to say this but the game was pretty mediocre. The graphics were great (although slowed down my laptop), but it kind of lacked substance.
I suggest more information for the towers, like damage, range, reload speed. The fact that the selling price isn't affected by the upgrades is a bit weird, as well as the same reward for different zombies.
With a bit of polishing it could be a very fun TD, the idea of having both turrets and different squads was interesting.