Please read this thread for a better description of the Heart of the underground fortress and the locations within.
The dungeon served as more than just a holding pen for those awaiting trial. Towards the upper levels of the Heart of the Fortress main chamber (with the giant dragon statue) there is a single port of entry with a heavy iron gate. The gate has been rusted closed for hundreds of years. There appears to be a hole broken through in the middle, but its difficult to tell if it was natural from the rust, or if something else managed to break the thick iron. Crossing through the hole in the gate and over the threshold, there is a dark and musty tunnel. The tunnel is long, and unpleasant, but it finally opens up to a large, low-hanging ceiling chamber reeking of hundred year-old rot and swamp water. Inside this chamber are a number of cells, each blocked by more heavy iron gates. Inside the cells are the remnants left behind by prisoners. Scratches on the walls, chipped stone, rusted shackles and chains still hanging from hooks on the walls. There are tufts of matted fur stuck to the sharp edges of the rusty bars. The ground is muddy, hard to walk on without sinking.
At the far end of the prison cell chamber is another gated tunnel. The gate remains open just enough to fit through without much danger or difficulty. The tunnel winds around at a steep slope. One misstep could cause a Pae to slide down the rest of the tunnel. The tunnel finally opens up to another dark, granite (no more mud), foul-smelling chamber. However this chamber is quite large. The ceilings are a few stories tall, there are platforms with ladders leading up to them on each story.
The true horror of this chamber lies in the artifacts contained within. Despite the hundreds of years of abandonment, some of the bones and bodies have been preserved rather well. Tacked to the wall on what could be described as the second story are several formations of wing bones. A learned Pae would recognize the bones contained in Angel and Demon wings. The formations contain the bones from many wings.
There remain tables with instruments of torture. Canopic jars line shelves containing hundred year old substances. Decaying bones rest in iron boxes. Swords, spears and other rusted weapons litter the ground. Mounted like a trophy on the far wall is the skeletal remains of a fish -- no, a fish-like creature with the torso and head of a Pae'il. On the third story are many plant pots. In these pots are small, dead saplings. Along with the potted plants are more grotesque displays of captured and tortured creatures tacked to the wall. These creatures -- giant bug-like creatures it appears -- are displayed in contorted and unnatural positions. Their wings are clearly detached and displayed beside the bodies.
An unknown light source appears and sheds light on the worst atrocity in this torture chamber: The decaying remains of a dragon chained to the walls high overhead. It is clearly attached with hooks that pierced the skin to the walls all around it in order to keep it stabilized -- and from escaping.
The stench of death, presence of tortured souls and gruesome discoveries are overwhelming. The details that remain of the pain a Pae'il had to endure are eerily preserved here more than the rest of the Fortress. There is even traces of blood, feathers, skin and fresh cut bones. How could findings like that survive hundreds of years under a swamp?