The web build is quite laggy. Doesn't mix well with rhythm games, which can require high precision at millisecond levels. The timing window for a full-scoring hit in rhythm games is usually somewhere around +-50ms, highly competitive games have windows of +-20ms for the maximum judgement, and some players are consistent enough to hit at +-5ms accuracy under certain circumstances. I don't know how large you chose your windows though. Did you even do it time based or is it position based?

The resulting patterning isn't particularly fun to play (from a rhythm game enthusiasts' perspective), and the notes aren't quantized to the music. But that's nitpicking at a high level. You could improve the gameplay by focusing on 4 towers that can be upgraded, which increases the complexity of the tower's rhythm. That way you have more than 4 "difficulty levels" in complexity (one whenever you add a new lane).

Here's the main issue: How does the player find out what the towers actually do? It matters during the building phase, but the game doesn't tell you. That's ok, in regular tower defense games a player can just look at the tower while the wave is coming and figure it out from the behavior. But the issue is that you can't do that, because you're busing looking at the bottom and hitting the notes. So you never actually find out what they do.

It's also not obvious how the performance during the rhyhm game affects the towers. Do they stop shooting, or do less damage?

Overall it doesn't really feel like a true "fusion" of genres, it's just two games you play alternatingly that don't really interact with each other. You don't think about the rhythm game while building, and neither do you think about the tower defense while playing.