Here we have the basic premise: our man is locked in a cell, ready to be punished for who knows what crime. We don't know how he's going to be punished, but things don't look too good.
The conflict here is mostly mental. Our hero has to wrap his head around what's going on. He must reassure himself that he is not yet dead, and he does his best to figure out where he's been imprisoned.
While trying to figure out the nature of his prison, our narrator trips and falls at the edge of a large pit. After eating what he thinks must be drugged food, he faints. When he wakes up, he finds he's been tied down to a strange contraption, above which swings a mysterious pendulum. Uh oh. But at the same time, this is when our hero slowly discovers what's up. He finds the pit, he sees the pendulum, and he figures out what his cell actually looks like. These revelations aren't comforting, though – especially when he realizes that the razor-sharp pendulum is slowly descending. Complication, indeed.
Here you have it – the moment of truth, the slow march of Time, the approach of death in the form of the razor-sharp pendulum. The narrator watches the pendulum descend, hearing the swish of the blade and feeling the air move with every swing. This is where you sit on the edge of your seat, clutching the book in your sweaty hands.
So, the rats save the day (by eating our narrator out of his bonds), but it's only now that the suspense kicks in. Think about it. Before, there was no way out: he was tied to a table with a razor blade headed for his heart. It's only when he has a chance – a really, really slim chance – of escaping that things get interesting. Poe gives us just that. And the question is: how the heck is he going to survive? (Which, by the way, we know he will.)
The denouement here is also another climax. Sure, some things are explained to us: what the room is really shaped like, where the light was coming from, etc. But, wait: the narrator also finds himself between burning and contracting walls and a giant pit of death. A pretty climactic denouement, don't you think?
General Lasalle saves the day, pulling our hero from the brink of the pit. The narrator is finally, definitely saved from the Inquisition. The Inquisition itself is finished, too. Of course, we know that our guy is safe from the very first paragraph – otherwise, we wouldn't be reading his story.