Love this idea for a take on a tower defense game.
It took a bit to see the verticality with the enemies that make use of height that only appear after a few waves. I like the idea of the stacking blocks, but didn't see too much of the point of the wide level structure, especially when cannons couldn't shoot past each other. I think cannons shooting through each other and enemies climbing over each other would be small changes that would lead to some interesting scenarios and make building in the earlier levels more interesting.