I played the game a little bit, nice work. :)

Here are some thoughts:

I liked the beginning of the level, but the spike sections were a little bit too hard...After a few tries I made it through the moving platform section, but then in the next room I fell down and hit the spikes that waited below, although after a few re-tries I completed this part too.

The placement of the spikes reminded me of some classic NES Megaman levels, but because the main sprite here is about two or three times as big as Megaman it is much harder to avoid those spikes.

All NES Megaman games have a lot of spikes, but that sort of level design won't work so well with a bigger player sprite. For example think about the level design in Megaman X in the Super Nintendo and compare that to level design of the older NES Megaman games...In Megaman X the player sprite was bigger than in the NES version, and so the levels in Megaman X had far less spike avoiding: mostly the spikes in Megaman X were located in pits or walls; it didn't have almost any "jump between these two spike balls"-style traps, because those kind of traps are too hard to avoid with a bigger player sprite.

But overall I think the demo level was quite well designed, I just died in the spikes so many times. :)

And also I noticed some small bugs, such as when you got a "game over", and after that try to start a new game, it just showed another "game over" right away.

And in one of the cave screens (in the place where you find the laser gun) the right side ladders seemed to act a little bit strangely: on top of them I pressed "up" and the sprite started "climbing" even though there were no ladders in that direction; and after that I could climb through the air and walls.

And also when I played with my gamepad, the running animation got "stuck" in the first frame every time when I pressed either "down" + "right" or "down" + "left" at the same time.

But those small things aside, everything looked quite good, I think it's a promising prototype demo game. And the end of the level boss was very entertaining, I loved the changing music, very Megaman style. :)

And the re-coloring of the Open Gunner sprites looks OK too; it's interesting to see these NES palette versions of them, and it all seems to work quite nicely with surt's tilesets.