I must say I'm not into the kind of games you're making here, but if you wanna listen to advice from a very lousy gamer I would say shorten the levels.
I was playing the first level and it seemed hard, but I managed to get past it. Now, when I saw the second level I immediately wanted to give up. The thing (IMO) that makes N addicting is that no level, at least the earlier ones, take too much time. In your game one level takes a lot, especially because you have to wait for the enemies to stand up and all.
I really liked the platforming and the sounds, I just think you could either add checkpoints or shorten the levels.
One idea that struck my mind is that it would be interesting if the character occupied 2 tiles instead of 1 (like the enemies) and managed to crouch, because trying to shoot in mid-air got pretty tiring.
I hope you don't take my criticism the wrong way, you can just discard it if you want, since I'm very
bad at platformers.