Pretty Good
But seeing that you've already made dozens of games, I know you can do better. The pure boss theme was good, but I think the hero could use hitpoints or something. My logic is that most of the bosses (excluding the first) have to be hit numerous times to die, while one hit--sometimes just on the feet--can kill the hero. Even with multiple lives, this is kind of annoying.
The puzzle in the second compartment has a shorter solution, but only deactivates the bomb if you use all pieces. I couldn't really get the lever, whether I mashed A as quickly as I could or if I went in rhythm with the icon. The walkthrough neither clarified my question or helped get past it. I had to watch it just to see the ending.
Still, the weapons/armor system is pretty cool, and I liked not having to slash through hundreds of minions just to get to a boss, as most side-scrollers demand.