it's a high quality game but i'm not a fan of the health = currency mechanic at all.
also i don't liek that you can't maze. you should implement a simple pathfinding mechanic. djikstra's algorithm would be fine for this. no need for any heuristics
edit: just a quick update. i came back to play again today (second attempt) and could have easily gotten to wave 30+ but got bored. i liked it more this time when i realized it was all about just spamming bombs and replenishing walls