I would like to add that from the beginning of the story, Montresor tries to convince us that he is not insane. When someone goes to such lengths to try and prove his sanity, it makes you wonder. Why does he bring up the idea of his sanity at all?
We also never know what Fortunato did that offended Montresor, so that seems strange. Montresor goes to great lengths to kill Fortunato, from getting his servants out of the house to chaining Fortunato to the wall and bricking it up. Most sane people would have a specific reason for a murder that was so well-planned.
It also has been fifty years since he killed Fortunato. It seems this has been an event which has eaten at him for this long period of time.
I would say that it's pretty creepy from the get-go, but when Montressor lures Fortunado deeper into the catacombs, allegedly to taste a rare wine and the descriptions become morbid and tomb-like, that's what does it for me.
Here's that passage:
"At the most remote end of the crypt there appeared another less spacious. Its walls had been lined with human remains, piled to the vault overhead, in the fashion of the great catacombs of Paris. Three sides of this interior crypt were still ornamented in this manner. From the fourth the bones had been thrown down, and lay promiscuously upon the earth, forming at one point a mound of some size. Within the wall thus exposed by the displacing of the bones, we perceived a still interior recess, in depth about four feet, in width three, in height six or seven. It seemed to have been constructed for no especial use in itself, but formed merely the interval between two of the colossal supports of the roof of the catacombs, and was backed by one of their circumscribing walls of solid granite."