I really like the premise of this game and the way that you place towers feels good and rewarding. I think the enemies could deal more damage or somehow be even more difficult because I found that I only took about 5 or 10 damage on average. I did notice some buggy behavior from the enemies that prevented me from progressing the game. On wave 2, the last two enemies got hit by something that made them fly off the screen's play area and so they never registered as defeated and I couldn't reach them. You could add a "kill zone" around the visible area that kills the enemies that make it out for whatever reason as a failsafe for that bug. Also to avoid it, a way back to the menu would help to restart if you get stuck.