Looking nice so far, I think that you need to play with the palette.
-The floor: try using less contrasting colors, the tiles are good but too well defined, you will lose your characters on it.
-The wall: try a lighter shade for the darkest color as characters may be lost here too
-The table: looks like it has a different style to the rest, the 'lighting' at the top and bottom is odd, use those colors and a defined light source to redo the shading/texture
-The light from the lamps is a perfect circle, it should light the floor differently if at all as it looks very flat at the moment
http://www.software-dungeon.co.uk/images/109974_dungeon3d.jpg http://t2.gstatic.com/images?q=tbn:B7qCNkrnX_WRnM:http://img244.imageshack.us/img244/7794/lionpitco9.jpg&t=1Take my words with a pinch of salt anyway, it will be nice to see characters in the prison and the are spruced up in general with more objects/structure.
As for suggestions to fill it up, if I think of a prison I think functionally what does it need?
- Maximum prisoners per area used
- Multiple levels of prisoners, and methods of accessing them - stairwells, lifts etc
- Prisoner showers, recreation area, food hall
- Guard/warden walkways to oberve prisoners and maintain order from
- Guard/warden barracks with their equipment, weapons etc, guard toilets
- Storage rooms for food, riot gear, recreation equipment, plumbing repair shizz
- Secure cells, secure compound walls
- Sick bay
- Isolation rooms
- Maximum security rooms for the most dangerous criminals
It has a castle-like feel to it here, so for nick-nacks/decals; shelves, high walkways, drawbridges/walkways, outer perimeter wall, heavy iron doors, archways, gargoyles, sewer gratings (large), open fireplaces, crates stacked unevenly, wall-mounted stuffed heads, wall mounted weapons, flags, tapestries, cracks/holes for mice, puddles/leaks, rubble, statues, torture chamber equipment, cellar doors (trap-doors) in the floor, treasure, skeletons, wall mounted shackles, nooses/excecution chamber, guillotine etc.
Some ref's you might find inspiration in;
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http://sp.life123.com/bm.pix/haunted-house2.s600x600.jpg-
http://chorrol.com/files/59/obliv23B.jpg-
http://amito.freehostia.com/Oblivion/Images/OB02-window.jpg