The game is very strategic on how to make the path for the enemies to reach to the next level without dying. I really enjoy how simple and easy to control. Everything is very smooth.
I also like the background music and sound effects. It has a unique vibe, especially when it comes to playing tower defense. Although this is backwards, it is still more than enough to give the vibe.
But yes, like Szym, the only thing I can say is that to add the speedup button. Overall, I'm impressed how you made the game. Great job!