Montresor carefully plans his revenge against Fortunato:
He ensures that his servants are out of the house the night of the murder so that nobody can report that Fortunato was in his house the night he disappeared.
He picks the night of the Carnival, a raucous evening of partying, when Fortunato's disappearance is unlikely to be noticed.
He ensures that Fortunato is drunk so that he won't be alert to danger. Montresor even takes liquor with him into the catacombs to keep Fortunato from sobering up.
He also never lets on that he is angry at Fortunato, so Fortunato has no reason to suspect he will be murdered.
Finally, he plans to commit his murder in isolated catacombs where nobody will hear Fortunato's cries for help.
Montresor's goals are twofold: first, he wants Fortunato to know that he is taking vengeance on him, and second, he wants to be sure nobody else discovers the murder. Because he chains and then walls up Fortunato, Fortunato clearly understands that what is happening is no accident. Further, because Montresor is telling the story 50 years after the fact, most likely as a deathbed confession, we know that he got away with his crime.
Montresor has his plan carefully worked out. First, he makes sure his servants will be out of the house by telling them he will be out all night, but he wants them to stay at home and not go to Carnivale. Montresor knows the servants will leave. This way, no one will see Fortunato go to Montresor's house. Next, Montresor uses Fortunato's ego against him by saying he needs Fortunato's expert opinion on some wine he has bought. Then Montresor baits Fortunato by saying he'll get Luchesi, another wine expert, to do it. Of course, Fortunato takes the bait and insists on going with Montresor. Montresor is very nice to Fortunato beforehand so Fortunato will have no reason to think that anything is wrong. Once in the catacombs, Montresor encourages Fortunato to drink more so it will be easier to chain him to the wall. Montresor then walls Fortunato in the alcove so he will never be found. Montresor is telling the story fifty years after the crime, so he is never caught and does committ the perfect crime, as far as carrying out the murder.