Interesting take and I was hoping to see the invincibility of enemies. However, when I played, clicking on enemies it didn't show me what they are invincible against (bug?). I liked the tower ideas and the amount of upgrades you can use to customize your towers, pretty good variety I have to say. I think I made some of my towers OP and was able to go through 20 waves pretty quickly. I did build all towers but I saw all enemies throughout the 20 waves take damage from all of them.
Overall fun game and was really cool. At one point (and due to having pretty solid towers) I was hoping I can stack waves together to see the mayhem. Might be a good idea to allow that. Good work!