Well this is looking really marvellous! I think you need to just work on tightening up the lighting and shadows and it will be excellent. Is this for any particular project, btw?
I'm assuming the light on the ground is a reflection of the light hitting the ceiling? If so I believe the whole ground should be much dimmer, and the highlight for it should be more saturated (I'm reasonably sure that would be the case, anyway). Aside from reflection, direct light from the lightsource pretty much shouldn't hit the ground, I believe.
I think the light hitting the ceiling should have a little more coverage (I'm looking centre-top here), rather than the patchiness it has at the moment...that would give the lighting on the floor more sense, imo.
I think there should be another subtle highlight tone being used on the stalagmites closer to the lightsource, to give more of a transition, or perception of fade-off to the lightsource, basically to give it more depth, and to lead the eye even more to that centre-piece.
The 2x1 dither present throughout alot of the dark zones is putting me off, I think you should rework it to a less rigid dither that allows for smoother and more varied and textured transition of tone.
The centrepiece (mouth, throne, altar, whatever it is) is a little rough in some places, work some AA in to really smoothen it and pronounce its form. Also, I think the shading on the
palatine process (excuse me, studying anatomy atm
i mean the "roof of the mouth") should have much more highlight and less shadow, I mean, compare it to the shading on the top of the stone bricks opposite it.
Also, I don't think the shading on the "nose" is accurate, it appears to me that it should be angled up, which means its primary lightsource would be light reflected from the roof, which I believe would mean the shading on the nose would be dimmer and more saturated. Same for the teeth.
Ok, its nearly 2am and I'm just back from work so that's all you can glean from me for now
. Good luck with it, it's really shaping up to be an excellent, moody, atmospheric piece.
EDIT: Oh also, you don't need to post a 2x zoom, pixelation has a built in zoom feature (clicking on the image)