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.