The game itself its great! Its a really fun simplistic tower defense, I would even say that is resembles a puzzle game in some way because of the importance of placing towers in good spots. The only issue I have is that the main mechanic, stacking towers, is not that useful. I have passed most of the levels without stacking. Its actually quite the opposite, when I tried stacking towers I failed miserably. Other than that the game idea is pretty fun, so with a little bit of balancing you could actually do something pretty nice! :)