Everything seems blurry to the narrator, especially at first, because it alludes to the fact that they are drugging him. They give him food and drink, but the narrator says "it must have been drugged" and after, he always falls into blurriness and sleep.
In the beginning of his imprisonment, he is put in the dark chamber, with no lights whatsoever, in the hopes that he would get up and walk around, and thus fall into the pit. He barely misses this fate when he trips and falls "at the very brink of a circular pit". So, that doesn't work. His captors, who open the door quickly to check to see if he has fallen, are disappointed. So, they drug him and tie him to a plank, in order to attempt the next torture: The pendulum of death. Fortunately, he uses his wit to escape that one too. But then at the end, it is alluded to the fact that the walls themselves are turning hot, almost aglow with fire. The walls have been set on fire, and the heat and air are so intense that the narrator "rushed to its [the pit's] deadly brink", tempted to throw himself over. Gratefully, he is rescued before he takes the plunge.
The narrator also notices that
The room had been square. I saw that two of its iron angles were now acute--two, consequently, obtuse. The fearful difference quickly increased with a low rumbling or moaning sound. In an instant the apartment had shifted its form into that of a lozenge....the closing walls pressed me resistlessly onward. At length for my seared and writhing body there was no longer an inch of foothold on the firm floor of the prison...
So the last torment is the narrator shrinking away from the pit, only to be seared by the hot iron walls, as they push him into the precipice.