What does the narrator of "The Pit and the Pendulum" discover when the light is finally let into his cell?

After literally wandering around in the darkness and just
by a stroke of luck escaping plunging into the terrible pit that lies in the middle of
his cell, the unnamed narrator sleeps, is given food, and then sleeps again through the
drugged victuals that he is provided with. When he wakes up again, he finds his cell lit
by a "wild, sulfurous luster," which allows him to see his cell and also to find out
whether his initial thoughts and assessments of his prison were correct or not. Note
what he discovered:

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In its size I had been greatly mistaken. The
whole circuit of its walls did not exceed twenty five yeards... I had been deceived, too
in respect to the shape of the enclosure. In feeling my way I had found many angles and
thus deduced an idea of great irregularity; so potent is the effect of total darkness
upon one arousing from lethargy or sleep! The angles were simply those of a few slight
depressions, or niches, at odd intervals. The general shape of the prison was
square.

In addition to having
two of his central deductions about the size and shape of the prison proved wrong, the
narrator sees that the walls of this prison were made of some kind of metal, covered in
frightening shapes and images. The narrator also sees how in the centre of the cell
"yawned the circular pit from whose jaws I had escaped."