I imagine that a fortress dedicated to evil would be built solely to create the maximum amount of pain, suffering and death conceivable. Ideally the fort should be filled to the brim will all kinds of gruesome architecture to enhance the horror of it all.
I envision a dwarven version of the Divine Comedy, with different sections of the fort dedicated to a particular depravity. Examples I can think of are starvation, dehydration, booze deprivation, pain (wooden whips I guess), madness. You could also have a whole bunch of different systems to kill your victims, such as drowning and magma chambers, cave in traps, monster pits...
I mean, why limit yourself to only one kind of evil when you can have it all? You could even throw in the elf and mermaid farms. The only problem I see is the limited quantity of victims...
Aesthetics wise, I would probably try to mod the game so I can build stuff out of the corpses. Imagine an elf flesh/bone fortress. Perhaps it would be possible in the new version. I would also try to make lots of blood to decorate the walls and floors of hell the fortress, and scatter skull totems, bone piles, rotting corpses and random spike pits all over the fortress.
For maximum perversity, there also ought to be some sort of demonic shrine where the rulers of the fort can offer sacrifices and such to live demons. Or maybe the rulers of the fort are demons.
Too bad I'm busy working on my current megaconstruction though.
EDIT: It would probably be easier to simply build out of red stone to make it look like blood splatter. You would also have more control over how the fort looks too.