Hi, I just played your game.
I noticed you said it was a prototype. I think the idea is pretty solid, but it needs balancing.
I managed to win the game by putting a double cannon on ground level and basically blew everything away. Before figuring out that I could add this cannon, the enemies too quick for the starting cannon or a different cannon to be effective.
It could be fun if things were slowed down just a little I think, and maybe alter the ground defence location, give it a weaker weapon or something you have to manually interact with with a different input.