Describe the room in which the narrator is kept in "The Pit and the Pendulum." How does he discover the shape?

is about a
man sentenced to death during the Spanish Inquisition, who awakes in the depths of a dungeon
cell.  At first it is pitch black in the vault, dank and cold, and the man can ascertain
details of his surroundings only by touch.  He assumes that the walls are made of stone, with
many corners and angles around, and that the room is fifty paces in circumference.  The only
thing he knows for certain is that the floor is damp and slippery, hazardous to walk upon.  Walk
upon it he does, however, and discovers a deep pit in the middle of the floor, a circular well
into which the Inquisition had intended him to fall.  Having discovered the plan, however, the
man retreats to the perimeter and is given a draught of some sleeping elixir.  Upon awakening,
he finds that he can see, and also that his assumptions about the shape and make of his room had
been incorrect.

In reality, the room was square and half the size he had
imagined, the walls made of iron panels, the ceiling about thirty or forty feet high.  Hideous
and repulsive devices to which the charnel superstition of the monks has given rise€¦overspread
and disfigured the walls.  These paintings are faded and musty as if with age.  The floor, the
narrator confirms, is made of stone.  It is only later, after he once again thwarts the
Inquisitions sick, slow, torturous attempt to murder him, that he notices why he can see:  the
walls are not attached to the floor, but are separated by a gap of an inch or so, through which
a dim light issues.  The significance of this detachment soon becomes clear, as the walls begin
to bend and tighten, pushing him in toward the well in the middle of the cell at the same time
as they begin to glow with an unbearable heat.  It is this latter fate that the narrator would
prefer over falling into the well, for in the light he can see what lies within €“ and within is
a terrible thing.