Hi there Spiritwolf.
I don't usually comment in the forum, but when I do I tend to comment a lot, so be prepared! 
This piece shows a nice work. I can tell you have put a lot of time in it, as well as a very good effort to light it creatively and in a different way. 
Now, the first thing it made me think of when I saw it, was a warehouse. Now, I don't know (i am just guessing here) that the light, shiny surface in the background is a long, frame-divided window; if it is, good, I could not think of it as another thing. Those windows (as well as what it seems to be the roof's supporting structures), made me think of the warehouse idea; I think this is a problem. It looks too much like a storage area rather than a prison, unless you were thinking of a temporary, unknown cell, that has been setup by a malign villain for his nemesis... but then, that would need some kind of storyline to go along with it, or some kind of background to give the viewer a better perspective and understanding of the scene itself.
The style is good, but kind of weird in a way. Don't get me wrong, I think you have a nice piece here and you can make it even better, but there is something disturbing about the lack of colour variety in what we could call 'the platform'. That green is a very powerful colour since it gives you 'space', ideas of unlimmited bodies of water (the ocean specifically).
I find that the stairway separates the upper part from it's environment (which it could be either way, good or bad, depending on what you want to achieve); still, they make me think inmediately in Star Wars, which is not a bad thing necessarily, but since it 'separates' the platform from the immediate surroundings, that makes me look at the platform as a strange object put in some kind of big room to which it does not belong. And directs my memory to Contact, since the structure looks similar in a way to the travelling machine they were building. Now, I don't know what your inspiration was, or what exactly were you thinking of while the conceptualization and elaboration of your prison, but I think you should first, settle down what you want the prison make the viewer feel and think of. Since it is a light/shadows exercise, you can use that 'medium' to transport the viewer, achievieng another, deeper, underlying goal. Try never think of a piece in terms of only being an exercise, or your piece would stop in the middle of its development and would never get beyond, to reach an audience in a deeper way.
Okay, let's go now to specifics. I think that for a prison, is quite full of light; if I was to be imprisoned there, I would think of myself pretty highly, not a miserable crimminal at all, but some kind of martyr climbing up to the sacrifice altar. Again, this is just my opinion though, and I am giving you feedback on pretty much what I feel and think of when I look at your scene.
Some suggestions...
1) Change the camera angle since it gives the appearence that, more than a filthy prison, is an intimidating torture chamber that is going to suck your brains out of your skull.
2) Change the background lights. It is kind of a nice morning for somebody to get tortured.
Try making them smaller, and putting them on an angle, facing down. Make it so as if the prisoner felt in a hole, and hopes every single day to see the horizon once more.
3) The chamber, I know it is supposed to be futuristic, but "futuristic" is not the same as "high-tech" (they can go together, but they are different). Make those pipes rusty and scratchy. If you focus at the floor of the platform, it shines pretty much, and it looks as if no cockcroach would dare to put feet on it.
I think I will stop here, otherwise thinking to much of those, might confuse you and change your idea's direction. I hope these suggestions help you out and give you more ideas to develop the scene further, but then again, it is your piece and your own style what counts. I would like to see more shots of it sooner. 
Once again, nice piece and hope you the best of lucks 
Eak
The Cat