Very solid! Nice gameplay loop, and pretty satisfying audio for the destruction sounds and music. If you decide to build this out more, here's a few suggestions for building further on the solid loop you have already:
Different enemy types: I think as it is now, the best strategy is to go full tall on one unit, as there arent any different enemy types that would encourage you to branch out into a variety of things, so after a while it might get a little repetitive. I just sat back and waited for upgrades most of the game, so it didn't feel like a super active role, which makes sense with the genre, but with more time I think adding branching choices would be a cool inclusion.
More in depth descriptions: Would have liked to have seen more description of what each of the different towers did, so I could make a more informed choice on what I want to go with.
Overall a really fun game and one I could see myself spending a lot of time playing idly. Good job! Really had a good time playing it!