There are so many to choose from in this gothic short story. It might be worth thinking about some of the following points that add a great deal to the amount of horror and fear in this tale:
1) The pit that the narrator only finds by chance when he trips over just before he walks into it when his prison is plunged into darkness.
2)The way his food and drink is obviously drugged so that he can be controlled and moved so further torture can be set up for him.
3) The description of him being tied up as he watches the pendulum descend.
4) The description of the rancid fat that he rubs into his cords that bind him.
5) The idea of the rats jumping on top of him as they gnaw his cords.
6) Lastly, the final section of the novel, when the hot walls of the prison press in, pushing him ever closer to falling into the pit.
What is key above all else is to note that these are tortures that are terrible and psychologically oppressive, as the narrator describes when he talks about the Inquisition:
To the victims of its tyranny, there was the choice of death with its direst physical agonies, or death with its most hideous moral horrors. I had been reserved for the latter. By long suffering my nerves had been unstrung, until I trembled at the sound of my own voice, and had become in every respect a fitting subject for the species of torture which awaited me.
It is the "hideous moral horrors" that the speaker suffers that make this tale truly terrible, and his description of himself as a person who has become "unstrung" is truly disturbing.