There are two descriptions of the dungeon given - one before the narrator has light by which to see it, and one after he he has been tied up and has light through the opening in the ceiling. In his examination of the dungeon in the dark, the narrator decides that the dungeon is made of stone, smooth and slimy, and that it is about 100 yards around. He also believes that the dungeon is circular and has a circular pit in the middle of it.
Here are the details from after light is provided:
"The whole circuit ...did not exceed twenty-five yards....The general shape of the prison was square. What I had taken for masonry seemed now to be iron, or some other metal, in huge plates, whose sutures or joints occasioned the depression. The entire surface of this metallic enclosure was rudely daubed in all the hideous and repulsive devices to which the charnel superstition of the monks has given rise. The figures of fiends in aspects of menace, ...overspread and disfigured the walls. ... the floor, too, which was of stone. In the centre yawned the circular pit ....Looking upward, I surveyed the ceiling.... constructed much as the side walls. In one of its panels .... was the painted figure of Time as he is commonly represented, save that, in lieu of a scythe, he held what, at a casual glance, I supposed to be the pictured image of a huge pendulum such as we see on antique clocks."