The foreshadowing in the first two paragraphs helps to create suspense. Montresor says that he "vowed revenge" on Fortunato, and he explains his criteria for how this must be achieved. He says, "I must not only punish, but punish with impunity [....]. I continued, as was my wont, to smile in his face, and he did not perceive that my smile now was at the thought of his immolation." Thus, we know that Montresor is going to do something horrible—something absolutely destructive—to Fortunato, as he refers to his enemy's "immolation." We also learn that Fortunato has no clue that he is in such grave danger, and we must wait to find out what horrors Montresor is going to unleash on his nemesis.
The revelation of Montresor's manipulative and cunning character also creates some horror. We learn that he is absolutely capable, intellectually and emotionally, to achieve the revenge he seeks. He tells us that his servants were all away from home: "I had told them that I should not return until the morning, and had given them explicit orders not to stir from the house. These orders were sufficient, I well knew, to insure their immediate disappearance, one and all, as soon as my back was turned." Montresor knows just how to fix things without arousing suspicion in his household. He also deflects suspicion from anyone who sees them together outside by wearing a black mask and a long black cape to disguise his identity.
Moreover, Montresor describes the nitre that clings to the walls in his vaults, calling it "'the white webwork which gleams from these cavern walls.'" Such a metaphor seems to compare the nitre to a spider's web, which makes Montresor, himself, the spider. This metaphor creates horror, indeed, because we imagine Montresor as a terrible predator and even begin to feel a little bit sorry for Fortunato, the prey who has absolutely no idea what's coming.