I like the escalation in this game. Simple but fun!

I liked the use of the theme to add a twist to the standard tower defense type of game. The enemies also move in interesting ways that makes the placement of towers matter more than I would expect from a grid layout. The best mechanic also feeds into the central positive loop that can drive the game to a quick end. Once enemies get through and take away health, it can be impossible to come back. A possible solution would be some sort of smart balance that can make easier enemies spawn for a bit after taking a big hit.
I encountered an odd bug that would take away a tower whenever I spawned a wave. I think it is a bug at least, I do not know why towers would be deleted normally without selling them. It would have been nice to know what the differences between towers are first, before finding out through trial and error.
Good job getting a good amount out of a simple idea!

Interesting to see a tower defense game made out of this theme