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.