I'm a sucker for tower defense games, and this one is nicely polished for being a game jam game! I'm impressed!
I did accidentally sell a few towers by clicking, holding down click, and releasing on accident. I would suggest that the "upgrade/sell" menu only pop up on mouse release, not on mouse down. But that's just me.
I think 42 waves was a ton for the first level. I felt like I had it down pat around halfway in. Though to be fair, I feel like a lot of the old, great TD games had too many waves in a lot of levels, haha, so maybe that's just me too.
Finally, I would consider coloring in the upgraded towers just a little more so it's more obvious that they're upgraded. Not a major critique, just something to consider.
All that said, I really like this one, and I think the three of you did an awesome job. Well done!