I really like the core idea and the way the game works. It's not dissimilar to the games I made a long while back, but personally I would remove this and resubmit when it's a bit more complete. I applaud how quickly this was completed, and it's great for that restriction, but unfortunately that doesn't mean that reviewers should rate the game any higher.
You also might have miscategorized the game--This is more of a twist on the scrolling shmup or bullet hell genres. Maybe something in the Action or Skill categories would be more appropriate.
I appreciate the instructions in the author comments, but it's really unclear what the player is supposed to do without them. I had to guess at how to start the game The music is beautiful, but I don't think it fits the presentation and fast pace of the game.
The sound effects are well-done, especially for the time limit--they're all unique enough that I know my shield is down even if I'm looking elsewhere on the screen, for example. Then again, the lo-fi sfx clash with the clear tones of the piano.