Nevermind, I got confused thinking it was refferering to the upgrades, I didn't realize there was 3 towers you could choose from.
Anyway, the mechanic is quite fun, though the process of switching towers is a bit janky, its almost impossible to switch towers on the later waves (since you have to remove the tower, choose which one you want to place, place it and upgrade tower to full again), also sometimes I try to place a tower but it doesn't do anything, so an indicator for that would be cool.
I also found some bugs when retrying, it once restarted the game at wave 10 lol, and another time I started the wave but no enemies appeared.
but yea I like the concept, good job!