The game ran fine for me.
I only had time to play the first two levels. Didn't have any trouble navigating. They were fairly easy, but I don't think that I can really give a good answer to the question of difficulty with the short time I had to play.
I did have fun playing and I really enjoyed the music.
I don't play a lot of platformers so I am not sure what advice I could really give here to improve it further. Adding some kind of visual effect when an enemy is destroyed would make it more satisfying to hit them I'd say.