This is a pretty cool prototype. I don't believe I have ever seen the card-battler "choice of 3 upgrades" mechanic in a tower defense game, before.
Would be nice to have a fast-forward button, and for the structure of the upgrades to be more obvious (ie. knowing which waves I will get a choice of upgrades) I think the progress bar behind the wave counter might have something to do with it. I think it would be clearer to either have a separate bar, or have the wave counter say eg. "wave 16/18", if upgrades are every 3 waves.
Do the rounds keep going forever? I got bored at wave 12, but kept playing until wave 20. ¯\_(ツ)_/¯