My progress

[DustyPorViva]

I think this is going in a very dark direction, and not for the better. You can achieve a gloomy feeling without making the player look at a near-black screen the whole time.

I made a very rough edit, and when I say rough I mean rough. I only edited a few parts, like some pillars, some portion of the walls, the stairs, the floor. I think contrast is something being neglected here. I upped the contrast in my edit merely by placing some black strategically. Also, the reason your stairs were suffering so bad is because they lacked a black line, and the step themselves(the underside?) were softened with your method of shading where a solid block of pixels makes it much more obvious. I didn't touch the door though, and the pits seems to have suffered from my color replacements.