The narrarator, a victim of the Spanish Inquisitionis, in a pitch black cell. The walls and the floor of his enclosure are moist and slippery, and appear to be constructed of stone. There is a circular pit in the middle of the cell. The walls are actually made of metal. He thinks it's a circle.

Eventually a light is turned on. He can see the cell is actually square. The walls are embellished with paintings of hellish figures; images of skeletons, fiends, and devils staring back at him. Worst of all, the narrator is now unable to move freely. He has been strapped and bound into a wood rack. There are rats everywhere and there's a big giant, sharp pendulum swinging above him, ready to cut him to pieces. The walls of the cell become hot at one point, and the narrarator describes the cell as smelling of hot iron.

The link below will give you a more complete description, but think square-ish, slick metal walls, big hole in the middle, and really dark. Eventually, our prisonor is able to see drawings on the walls and ceilings, and, of course, he sees the pendulum blade descending as well. At the end, he learns that the walls can also be heated and are moveable.