there sould be a morter/artillery tower/weapon which can shoot OVER other towers. Also you should be able up towers. + so the dev. sees this and it willl happen!
A good Zomibie game for somone to make would be one based off of the book, Zombie Survival Guide By Max Brooks. He gives a very realistic scenario of an outbreak and instead of the zombies reanimate, it takes 15-24 hrs dependent on the person and they can only shuffle, buy the moan to signal others... my opining this game is great and you should base the 3rd of of the book