Music is great, but sfx are a little to loud and starts to bother after some time.
But, let's get to the main subjects:
1) Your artstyle is good, but I think t would shine if you start working with hgher resolutions. You made a lot of assets, but sometimes in gamedev less is more too. I'd recomend you to make less types of similar assets, but making each one more detailed and add more frames to animations.
2) There is a room for improvement about your level design skills. Keys hidden all over the map in tight spaces isn'ta good practice. At least not with this sort of a camera. If I don't know how many keys I need and I don't see all of them, I'm starting to feel a little bit of unfairness.
3) This point is tied to the last one. You need to work on your enemies. You couldn't make them interesting, so you tried to just add a lot of them. But again, this isn't a good practice. Most of this emenies are quite unfair. Enemies which are following you are cramed in tight spots making it almost imposible to dodge without colliding to another enemy. And the sooting enemy... Well, just make projectiles collide with the walls, because otherwise it feels like damage out of nowhere on th other end of the map.
4) Game is too easy and unbalanced Can rush almost every level with a good RNG. Haven't found any use to space bar ability. Make it kill enemies in a greater radius and it'd be a lot more useful and limited usage will make sense.
5) I recommend you to add tutorials to your games. Just overall.
Didn't get how the less is more in our game.
Don't take this comment to close. It's a good game for a beggineer and I'm not trying to offend you. Just showing examples of areas for you to improve.
Have a nice day! Good game!