theres something here!!! i think its a really cool idea.
it would be nice if the turrets fired faster (so like the overall loop is faster), even if they were weaker in damage to compensate, that way it can feel like you're actually building a beat. this feels like the kind of game that you could unironically jam to while playing. the issue is i think the tempos just too slow to actually sound like music.
I think that playing back the sequence is a cool idea, its difficult to actually perform though, especially if you want to look at the enemies that are on their way. Multi-tasking that is pretty tough, and whenever I play music I can only really multi-task like that when its a song ive memorized. Maybe if the towers attacked automatically to the beat, it would lessen that mental strain and also make the game easier to play, and then the focus could be more on the music and the towers, rather than timing.
A friend of mine suggested making the enemies spawn in waves too, at the start of every loop rather than continuous. Also was concerned that there was really only one effective combo of turrets.
would be interested in seeing where you take this in the future, good luck, and dont feel like you need to implement any of those suggestions. seriously no pressure.