Why do you think Poe chose to bring together the dreariness of the catacombs with the festivity of Carnival?
He did this to juxtapose these two situations with the manic nature of the character of Montressor. He describes the scene as the "supreme madness of the carnival season." Montressor vascillated between being very manic and extemely dark. There was humor thrown in with the darkness. Light and dark-black humor no doubt.
By using the setting of the carnival against the catacombs, the horror is heightened. The contrast is absolute. Fortunato is so happy at the festival, he is carefree, enjoying himself, so he is represented by the gaiety of the festival. The catacombs, on the other extreme represent Montresor, who is gloomy, not celebrating and hiding a dark and deadly fate for Fortunato.
Poe is so interesting to read because you can find interpretations in layers in his works. He pulls his plot, theme and conflict together using such great imagery. His works come alive in the mind when you read them.
Fortunato's, "good luck" has run out, he is prideful and unaware that he is being led to his death by Montresor, who is as cold as the catacombs in his calculating the means of Fortunato's demise.
I agree with all above. In addition:
For one thing, he felt his victim deserved such a horrible place as his deathbed, and for another, the screams would not likely be heard over the pomp and ceremony of the carnival.
Have you seen the medieval paintings depicting the danse macabre? That's what Poe has given us.
Because of plague, war, and poverty people of the Middle Ages were much more aware of death than we may be today. Medieval artists recognized that even in the midst of gaiety, death is ever-present. That's why the style called the danse macabre began. In these paintings, the artist would often depict a skeleton or two among the people in their paintings. These skeletons would appear to be smiling, as if they were mocking the living.
Here's a web site where you can see pictures and read about it:
I think the contradiction and the masquerade that is part of the carnival serves to underscore the idea that we never know what a man is thinking or feeling. Monstresor commits this murder for reasons that seem to be of his own mental creation. He is never caught and goes to his grave unrepetant of his sin and, as Poe suggests, considered an honorable man. In other words, every man has a mask that conceals the horrors underneath.
The two settings offer a contrast, so that the catacombs become just that much more frightening. Also, since it is a masquerade, the characters are in costume. How fitting that Fortunato is dressed as a jester or fool because he is fooled by Montressor about the Amontillado in the catacombs. Montressor wears a black cape and black mask... much like a thief or a bandit. The color black is symbolic in that it signifies evil... and this characterizes Montressor. The jester's outfit is significant because of how easily Fortunato acts the fool in deciding to follow Montressor. Several times Montressor gives Fortunato an "out" by saying that Luchesi could taste the Amontillado, but Fortunato's foolish pride keeps him going, even to his death.
Poe is notorious for juxtaposing his settings in the story to really bring out the flavor of his horror. The festivity of the carnival allows the reader to really experience the surprise in his revenge plot the way that Fortunato does. Fortunato was taken completely by surprise. He believed he had run into an old friend, Montresor and that Montresor trusted his connoisseurship of fine wine and was taking him for a rare taste of the amontillado. Poe uses the setting to foreshadow the horrific events to come. It is not by accident that the "amontillado" is in the catacombs which were used as a place to store wine, but their primary use was as tombs for the estate's family that had passed on. The dreariness lets the reader know that something foreboding is about to happen.
Consider the physical setting, What is the significance of the transition from street to catacombs?