Cool concept!
I enjoyed the way you introduced new enemies and then created different situations combining them in many clever ways! Also, thanks for putting in a secret room with an HP upgrade (I initially thought there was a glitch on the tiled map, but that was a nice surprise!)
The game length helps you taste the idea and not get too bored with it, as giving the ability to fly from the beginning pretty much breaks most classic Metroidvania game progressions, making it too easy.
And there seems to be no reason to map actions to the mouse button. Putting them back on the keyboard where they belong will give you extra accessibility credits for free ¦3