For a 48h jam, this is extremely polished, great job on the graphical style and overall mood/flow (the music and notes playing during the build phase is a great decision). I also appreciated the visual communication of the game, making the game quite intuitive eliminating the need for a tutorial (apart from different tower abilities).
Some enemy variety would be nice, forcing you to actually use all different towers, and i found the game to be repetitive quite fast (after about 3~4 Waves). I missed some kind of goal to work towards (score, last wave, boss, etc.) to keep me going.
Balancingwise i felt like the rythm part was quite a bit more important than the tower placement/postioning, meaning the waves are quite easy as long as you hit the notes, although I assume balancing this would take a significant amount of time.
These are just minor things though and a bit out of scope for this jam, everything that was implemented was implemented well and had a purpose, so congrats on this entry!
Also, you can stack towers on top of each other :)