The art looks awsome. The fire balls, tourches and the green enemies looks awesome.
The first thing that could be improved is the camera. The camera moves around very quickly and ut doesn't feel smooth. You should som kind of smoothness.
The background art and the spike art looks kind of bad if you compare it to the other art. As you can see with the player and fire balls you can draw pretty well. So the only thing you have to do is to put more effort to the background and the spikes.
When you enter a door it doesn't take you imadetally to the next level because it needs to load it first. This leads to a sudden pause in the game when you enter a door. I suggest to add some sort of scene transition. There are many tutorials out there on that so it shouldn't be a problem.
All green things will damage you, or at least a thought. I thought the green jumping thing will damage me so I tried to avoid it but it doesn't. Your game broke the rule by making a green thing not take damage. You should change it up.
The last thing is the hears system. Sometimes the enemy takes two damage other time none but for most times it takes 1. It is really annoying to lose two hearts because one enemy touches you ones. You should try to fix it, it is a simple fix.
The level design was one of the best I have ever seen. I loved the level where you have to wall slide and shoot the enemies. I felt like a total badass shooting those enemies. The game has a lot of potential
In conclusion this game is really fun and with some more effort I think this has a lot of potential. The music and the art is gourges and hitting an enemy mid air feels amazing. You should really update this game and put in all your effort. This was super fun to play and I want to play it again some time. Let me know if you update it.