As a start, I placed a Solid cube to be the foundation. On that, I placed a VisArea and took any ambient light down to zero. There were to be no light to begin with. I then moved the VisArea out of the way and placed most of the objects, as it is simpler when aided by the daylight.
Then the VisArea was moved back and a key lights were placed and tweaked for mood. From that, the dungeon sprawled from the cell outwards to the tower.

It is made entirely of original objects, and objects that can be used in many different ways. All foliage I made up, and does not exist in real life. I took a look at different plants and came up with what I thought would fit the scene - gave them names, and so forth. It is, after all, a fantasy world.

To me, it is important to have some story behind everything. Also, something I find important is that not all objects have to fight for the limelight. The eye has to have rest. Sometimes things are just bland, and it is, in my opinion, befitting of how it is in real life. Not all objects are adorned or have scrolls. In this case, I pictured that a lot of the dungeon itself was made by orcish slave labour, which in turn would add a bit of slip-shot to a lot of things, while other things simply would be utilitarian.
Also, there was no master architect at work for a lot of the complex, but parts of it show signs of design.

This is the cage from the cell area. It is basically made of sculpted iron bars - some ornamented - something akin to viking snakes, for some visual interest. The high-poly is then projected onto a cylinder, and afterwards lightly shaped with a Bend Modifier. The top and bottom are solid, as it is possible to look through the bars. All the iron bars are slightly flared outwards, to give the illusion of thickness that otherwise would be lost on a normal map.

The next is an example of a high-poly moulding piece for the sewer area. I have copied the piece across, made some small adjustments, and then made one, large low-poly mesh that then could be bent to fit within the sewer area. Like most pieces I make, I start with a rough shape, made in 3DS Max. I divide it for equal poly distribution, and sculpt it in ZBrush. It is then brought back into 3DS Max, and a low-poly mesh is made. I always use 3DS Max's Render to Texture for normal maps, as I find it has the best results, and that you have the most control.

Following is the low-poly of the light droid that is used in the scene, as well as a texture example. The texture is, as most of them, entirely painted in PhotoShop. You can also check out an example of the high-poly sewer pillar.