In "The Pit and the Pendulum", what happens to walls of the prison to force him into the pit?Why does everything seem like a blur to narrator?

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.