In "The Cask of Amontillado," Edgar Allan Poe creates tension out of his use of dramatic irony. In the story's very beginning, his narrator, Montresor, vows vengeance against Fortunato, giving the reader insight into his motivations. Yet Fortunato himself remains ignorant of Montresor's designs, continuing to trust in him even as he is brought deeper into his enemy's web.
I would suggest that this tension is key in Poe's creation of suspense. With that being said, however, keep in mind that Poe does not reveal everything to his readers right away. While Montresor's malice is clearly expressed throughout text, the specific details concerning his vengeance are withheld until the end. This only creates further tension in the story. We can assume Montresor plans to exact some monstrous revenge against Fortunato, but we don't necessarily know what that revenge entails.
Poe sets the suspenseful mood of the story by establishing the conflict in the story's first line, in which Montresor announces the reason he will exact revenge upon Fortunato, though Fortunato's "thousand injuries" and final insult are not made explicit. The outcome is never in doubt if the reader considers that the story is told in retrospect, with Montresor satisfied that he has achieved what he set out to do.
The story's setting also builds suspense. Since it is carnival time, all are in costume; this provides a reason for Montresor to be dressed not unlike a grim reaper and for the unsuspecting Fortunato to be in a clown's garb. As evening settles into night and the men descend deeper and deeper underground, suspense builds.
Poe utilizes a darkly humorous foreshadowing when Montresor claims to be a member of the Masonic order and then wittily pulls a trowel from his cloak. Additionally, the fact that Montresor's family crest is described with its symbolic and explicit motto, "No one harms me with impunity," bodes ominously for Fortunato.
Readers begin to recognize that Fortunato's increasingly drunken state will contribute to his undoing. With each draught of wine offered by Montresor, he inches closer to his doom.
Poe builds suspense starting with the first paragraph, in which he has his narrator tell us he will get revenge on Fortunato but doesn't tell us how or why. How has Fortunato wronged him? What will he do to him?
Suspense builds as we enter the cold, damp vaults beneath a home where all the servants are gone for the night. It grows as we enter, with the characters, a smaller crypt, "lined with human remains." We worry, for we know Fortunato is drunk, and now isolated from all help. The upside-down frame of the Carnival, a time when normal restraints are cast off, is a literary device which also adds to our sense of unease.
The narrator chains Fortunato to a wall, but like the victim, we still don't know exactly what is going to happen: we are still in suspense. Then, rather than just telling us he has walled up Fortunato, he takes us through it step by step, as if in slow motion: "I laid the second tier, and the third, and the fourth; and then I heard the furious vibrations of the chain."
The narrator continues to drag out the work of the walling, while revealing that nobody can hear the screams of Fortunato. But will the narrator actually go through with his plan, or is his revenge simply to play a cruel joke? We are in gradually lessening suspense until the last brick is mortared in place.
We never learn what Fortunato has done, if even anything, to warrant this fate. But by using the technique of carrying the reader step by step through the grisly scene, we are kept in suspense as to its final outcome.
There never is any doubt that the narrator will exact a cruel revenge. The suspense lies not in the "if" of the revenge but in the specifics of "how" it will occur.