This is a cool survival game! I really like the tap and hold mechanic. It's deceptively simple, but the strategy lies in timing the pulses correctly. I can see it being extended to different patterns or even different notes/buttons if this game gets further development.
Other folks have already mentioned it, but it would be nice to have some sort of indication of how long to hold the button for the long note. I feel like it's important for when different types of enemies attack simultaneously so the player can sequence their pulses correctly.
Excellent work on this submission!