The contrast in the two primary settings of "The Cask of Amontillado" is especially powerful in contributing to the confusion of Fortunato and the eerie mood that Poe creates.
Fortunato encounters Montresor during a carnival. American readers might liken this setting to something akin to Mardi Gras: wild festivities, an abundance of alcohol, and generally convivial crowds. Fortunato is ready to fully engage in the festive atmosphere, as is evident by his dress:
The man wore motley. He had on a tight-fitting parti-striped dress, and his head was surmounted by the conical cap and bells.
Fortunato also prides himself as a wine connoisseur, so this scene would have particular interest for him. Ready to engage in all the Carnival can offer, he is not skeptical of a "friend's" offer to take him to investigate the authenticity of some Amontillado.
As Fortunato leaves with Montresor, the setting changes quickly—and so does Fortunato's health. He begins coughing as they stand on the "damp grounds of the catacombs of the Montresors." The setting is no longer festive and jubilant but ominous and foreboding. Fortunato's body seems to react to this drastic change in setting in a way his mind cannot grasp. Montresor actually provides him with an opportunity to leave, telling him,
We will go back; your health is precious. You are rich, respected, admired, beloved; you are happy, as once I was. You are a man to be missed.
Yet Montresor gauges Fortunato's personality correctly, knowing that his pride and curiosity will lead him deeper into the catacombs instead of choosing the option which would have ultimately spared his life. From there, the setting grows more eerie. The moisture hangs on the "bones" of the catacombs as they descend again and again until the air is foul.
This descent represents the final fall of Fortunato. Led to his own tomb by an unforeseen foe and his own pride, he is forever cut off from the free-spirited and joyous world above him.
The setting of Poe's macabre tale is important to the plot because it happens during the often crazy and unpredictable time of Carnival. In Italy at the time, Carnival was a riotous festival which came just before the religious observances of Lent and Easter. During Lent, practicing Catholics will fast, giving up meat and alcohol for up to six weeks. Fittingly, Carnival is defined as a farewell to meat (carne=meat and vale=farewell). During Carnival, meat and especially alcohol are in abundance. Participants also don masks and wild costumes such as the "conical cap and bells" worn by Fortunato.
Montresor carefully chooses this as the backdrop for his sinister plan. First, he knows that his servants will not be around as witnesses, because they will be busy indulging in celebration. Second, he knows that Fortunato will have been drinking and not in full control of his capacities, making him easier to lure into the depths of the catacombs. Montresor boasts at the beginning of the story that he would get away with the murder of Fortunato. The fact that the entire town is preoccupied aids his success in this endeavor.
The setting of "The Cask of Amontillado" is important because of its emotional effect on the reader. Montresor lures Fortunato into the gruesome underground catacombs with the promise of a delicious wine. Poe is luring the reader into this same setting by arousing his curiosity about what is going to happen and at the same time making the reader want to see that cask in his imagination and taste it vicariously. But in order to follow Montresor and Fortunato, the reader, in imagination, has to travel through that dark, evil-smelling setting full of human bones. The setting is part of the total experience of the story. It is almost as harrowing as Dante's descent into hell or the earlier descent into hell by Odysseus.
Poe begins the story up on the streets where everybody is making merry. The reader doesn't know what he is in for. He is gradually drawn into Montresor's palazzo, down a flight of stairs into a wine cellar, then along a series of darker passages full of human bones, and finally to the site of Fortunato's immolation. The reader can appreciate the full horror of Fortunato's fate because he can now understand what it would be like to be left down there forever.
Poe also had a plot problem. He wanted to keep Fortunato from talking because his intended victim would surely be asking questions about the Amontillado. Who did you buy it from? How much did you pay? Have you told anyone else about it? Why haven't I heard about the shipment? Why did you store the cask so far from the bottom of the stairs? Where are you taking me? Fortunato could become suspicious if Montresor could not provide satisfactory answers to all the questions he might think of. And Poe has established that Fortunato knows more about Amontillado than does Montresor. Fortunato could ask questions that neither Montresor nor Poe could answer. So Poe provided Fortunato with a bad cold and a cough, making it hard for "his poor friend" to talk. But if there is going to be limited dialogue as the two men wend their way through the catacombs, then Poe will have to fill the space with description, which he does.