Really nice mechanics and level generation! Loved the implementation of the theme.
The enemy variations made the levels feel really different each time I played---I didn't even realize I am in the same map for half of my gameplay.
One small issue I faced is that I don't exactly know if there is a regeneration factor. In addition, I don't know how to use the item I bought in the shop. I suggest if a main game tutorial isn't there, you can always put the informations on the game page!
Great job!