Cool game, and pretty tough as well!
I liked the idea of a reverse tower defense and found the game had potential for some strong complexity. I had fun trying to figure out some kind of winning strategy.
To give feedback I'd suggest improving the readability in your UI elements. For example, in the main menu, in the "How to play" section, I thought I was stuck and was trying to click the green bar to progress, unaware the tombstone in the background existed. Similarly, during game play, the surrender button wasn't too clear either and I pressed it without realizing it'd end my run, some kind of warning would do a lot of good.
Also, I did find a bug where I was able to purchase a mana upgrade worth 2 using only 1.
Great work! I'd love to see it updated in the future!