Throughout his short story "The Cask of Amontillado" Poe employs dramatic irony. Remember, dramatic irony is when the audience or reader knows details that the characters on stage do not yet know. As the reader, we know that Montressor is planning on seeking revenge against Fortunato, but Fortunato is unaware of this plan. Montressor begins the story by talking directly to the audience and delineating his plan to get his revenge.
The thousand injuries of Fortunato I had borne as I best could, but when he ventured upon insult I vowed revenge. You, who so well know the nature of my soul, will not suppose, however, that I gave utterance to a threat.
Throughout the story Montressor exploits Fortunato's pride and greed and uses it to entice him to come to his catacomb and check out his Amontillado. Again, this is dramatic irony because the audience knows Montressor is don't really planning on visiting Luchesi, instead he's lying because he knows that it will only make Fortunato more interested.
“As you are engaged, I am on my way to Luchesi. If any one has a critical turn it is he. He will tell me—”
“Luchesi cannot tell Amontillado from Sherry.”
“And yet some fools will have it that his taste is a match for your own.”
“Come, let us go.”
“To your vaults.”
“My friend, no; I will not impose upon your good nature. I perceive you have an engagement. Luchesi—”