This is probably one of the best things I've played all year. The action is fast paced, the guns are punchy and they all have their uses, even the pistol. Also, the low-fi pixel textured environments are really nice to look at in that.
I played on normal and the difficulty was spot on for the most part.
If I had any suggestions, maybe have some way of staggering the bigger enemies or breaking their charge. Maybe make some of the more powerful weapons like the double barrelled shot gun and revolver?
Also, I'd love to see more outdoor levels. I think the outdoor section was probably my favourite.
One last thing, I found 2 of these little maps, one was out in the open and the other in a secret passage. I thought they related to some kind of puzzle in the big prison room right before the yellow door but if it was I couldn't figure it out and their didn't seem to be any feedback on anything I tried.