you need to do something with the turrets. they are a major waste of time and money and are unnecessary to beat the levels. instead of moving from room to room so quickly you should stay in one area for a while and at the end there should be a large wave of zombies. that will encourage players to do more than just station there characters somewhere and wait out the waves. the game is also quite laggy at times.
i'd love to play this this game but the unit AI is autistic. the archers run in attempting melee while melee units will walk back and forth when ranged units attack them. it looks great but its far too frustrating to enjoy :(