a key to build multiple towers would be nice, but apart from that it is a good game. simple but good graphics, nice gameplay, not to easy and not to hard, and unlockable stuff.
still I don't get one thing about this sort of games: if the city was attacked by zombies the shopkeeper will probobly give you his weapons for free if you save him, so why should you pay?
I loved the concept of the game. and it was worked out quite good as wel. but I've a few tips to make it even better:1 hotkeys,2 an epic boss at the end, 3 upgradeble life, mana and base damage.