Great idea. You can do a lot with this, without adding anything, per se. It has a strong composition, with the balance/tension on that diagonal. First off, rescale your stone texture. It is too tall. Next, unless that light is NOT coming from the sun, you need to either use a distant light, or pull the one you have way back. The lit area from the window should not be larger than the window itself. Make sure your ambient light is way down, or off, or you will not get the dramatic lighting you want. Next, either play with the radiosity settings like you were, or control it more by using more lights. For instance, you may want to place a point light with falloff near that chair, to enhance the bleed of the light. The question that pops into my head when I see the pic is "Where's the person?" You may want to provide some sort of answer, even if you do not want to put a person there. You know, something like a skull in the corner (or maybe you have more profound ideas}. You could even have one of the bars be sawed through, or place tic marks on the wall counting days... etc.. If you subdivide that mesh, then use weight maps to control your textures, you can add some darker dingier detail, like in the corners. This is, of course, just my opinion/advice. Hope it helps, but you're the artist.

BUMP MAP on the stone!!!!!!

I think that some of these stones might actually need to be modeled, specially around the area of the window - if only to cast a more interesting shadow. Not sure what's happening on your shadows, but the real sharp edged shadows aren't working as realistically. I'd try faking this effect with an area light instead. Still, it looks very dramatic. :o)

Thanks a lot guys - I really appreciate the help. I'll have to do some homework and find out how to accomplish some of your suggestions, but hopefully I will have an update in a few days.

i know it would take ages, but if you modeled all of the stones, sorted out ure textures and shads a wee bit, i think you would have a really good image on you hands. you could just model and texture a couple of bricks and just modify them a bit for every brick. (i think it would be worth it) ;-)