To beat the last level you move your base right next to the tech upgrade, then span out collectors over that top level and make 3 or 4 mortars to make sure that the creepers don't overrun you base. and then it is pretty easy from there
good game, i ended up establishing a constitution, finding a cure against zombieizm, closing the evil zombie portal, and claiming every city block. 5/5!!
 "Tripod Attack"){kind=icon}
 "Infestor"){kind=icon}
 "Creeper World Training Sim"){kind=icon}
 "Rebuild"){kind=icon}