Personally I'm a tower defense freak so I dont really know if my assessment is kinda screwed, but this was a fun game for me that I enjoyed. The effects and simple, yet clear art + sound design makes it really enjoyable. Even though the game was short and enjoyable, I hoped that it had different towers or upgrades.
I think more enemy variety would be great , even if it's just a palette change for the design. Take that in combination with enemy placement would have been the best since it kinda gets easy in late game since you can just spam tower (there were a lot of good spots for towers too).
Besides that maybe you should turn the alpha of the tower range line when placing towers in order to make it easier to see when towers are clumped together in late game. Outside of that as I said, I really like the game!