The mood or atmosphere of this story is one of foreboding, fear, unease, and anxiety. This dark mood is conveyed through both the setting and the dramatic irony created by the words of the narrator. 

The setting is what Freud, in his essay on the uncanny, would have defined as unheimlich, meaning eery or unhomelike. Most of the story takes place in the dark, damp, isolated catacombs beneath Montresor's home. Catacombs are where the Romans buried their dead, so the bones Montresor and Fortunato pass act as eery harbingers of death. Beyond that, it is the night of the carnival, a time of revelry in which the ordinary rules of behavior are suspended. The bells on Fortunato's party cap remind us that we are in a topsy-turvy world where all is not homey and normal. Darkness, isolation (the servants are all out), and the bones of dead bodies make us anxious and fearful.

It is called dramatic irony when the readers or audience of a work of literature know more than some (or all) of the characters. As readers, we know what Fortunato does not. We know that Montresor is planning revenge on Fortunato. We know that Montresor has a carefully laid trap, though we don't know what it is. We know Montresor, with ill intent, has lured Fortunato into the catacombs and is keeping him drunk. We know that something bad is going to happen. This builds an atmosphere of foreboding, unease, and fearful anxiety. 

In Edgar Allan Poe's classic thrilling story "The Cask of Amontillado", Poe does a great job setting up a dark and suspenseful mood. When you are analyzing a story and looking at the mood, you must not get this confused with tone. The mood of a story is the feeling that the reader gets from reading the story. The tone is the feeling the author has toward the subject of his or her story. Poe brilliantly sets up the mood of this story by using multiple elements that all help to feed into the same feeling. Which one of these elements is most crucial to creating that mood can be debated. I personally feel that the setting is an incredible influence on the mood. He sets up the mood using the setting  by having the characters walking through the dark catacombs which were "insufferably damp" and "encrusted with nitre". He points out the "white web-work which gleams from these cavern walls". The narrator describes how he "took from their sconces two flambeaux, and giving one to Fortunato, bowed him through several suites of rooms to the archway that led into the vaults." All of this sets up the suspense as they pass through the dark catacombs, descending into the unknown future, surrounded by death.