While the pixel artist here has skills I don't like how so many people are obsessed with imitating the Cave Story style. I don't mean to sound unfair, but when I see this style I feel like I'm playing a Cave Story fangame. Idea here I guess is decent, but somewhere in the line of cliche seeing how there's plenty of "dark/flipped/mirror world"- kinda ideas as of late, for one it's not a very spectacular experience.
As for performance maybe the most pointed out fact, it's awful for few of us, hence we should assume there's something wrong. That noise effect might be the cause of this. Not only it's a flash killer, it's also a very easy and overused way to express distortion, darkness and such, which I believe you went for. Also as a vivid pixel art fan and hobbyist I find it a very cheap and dirty solution.
Music is well composed, but needs variety. It gets repeative and boring already after hearing it three times. Tell you what though, the atmospheric change when you go to underworld is a jackpot.