From the Introduction:
A fiery moat divides the room into two. A hip high wall on either side of the moat should keep anyone from falling into the molten rock slowly flowing along below by accident. A sturdy wooden drawbridge, which is raised at the moment can be lowered across the moat. Large cells to either side of the entrance are separated from the rest of the room by sturdy wooden, iron reinforced doors and rusty iron bars.
Like in the rooms you have left behind, the walls are built from solid granite blocks, set so finely that hardly even a sheet of paper would fit between them. Numerous columns built from the same stone help distribute the enormous weight of the rock above. Besides the fiery glow from the moat a small number of rusty lamps shed a little light in the room.