I really like how you implemented the Nuclear Throne-like mechanic. Weapons could give you some feedback on how long they will take to be reloaded, but the overall feel of movement, shooting sounds, particle effects and variety imprint intensity. So pushing the trigger feels beyond playing a sound and creating a bullet; it kinda made me relate to the character, feeling the heaviness of the tool and its recoil.
However, I had trouble advancing to the next stage. Was I meant to do something in particular? I kinda felt stuck and had no idea why; so the map might make this things more clear.
Anyway, they art is rad, and combining huge pixels with smoothness of moves makes it feel bold, I'd even say dirty as war is. So I'd say it has a great load of immersion and it is quite promising, if you want to extend it to a longer experience. I think you got the basis for a great game: fluent, good looking, action is felt not just shown...