I really like the concept, but the update mechanism needs a little more 'freedom'. Also, there is no reason to focus on more then 1 type of zombie, effectively limiting the game.
Something i found confusing, or irritating more, is that the fire speed upgrades tell me they 'decresase' the firespeed further. Although it's obvious what is meant here, the FIREspeed should increase, what decreases is the reloading time. :)
Check the steam, follow it until you come to a wheel, click it, rinse and repeat