Many questions have been asked about the significance of Fortunato's carnival costume and the jingling bells on his hat. In the first place, Poe dressed his character in a jester's costume to make him as conspicuous as possible. Fortunato chose the costume because he thinks of himself as a clever jester. Montresor wants to lure Fortunato off the streets and down into his catacombs. The fact that Fortunato is so conspicuous and even has ringing bells on his hat seems to make Montresor's task more difficult, since Montresor does not want to be seen with the man on the night he disappears. But this is the main plot problem to be solved. The protagonist, Montresor, has a motive, which is to commit a murder. His problem is to do so without being caught and punished. The victim's costume simply makes the problem greater. But Poe, a literary genius, realized that the more attention Fortunato attracted to himself, the less attention would be given to the man in black who was with him and who was wearing a black mask. Many of the intoxicated revelers would remember seeing Fortunato, but none would remember seeing his companion, who would be almost a shadow.
So the jingling bells help to attract attention to Fortunato. But that is not all. They serve to place him, to locate him, to represent him while the two men are down in the stygian catacombs. The reader eventually cannot visualize Fortunato in that darkness but can only hear him and know that he is following his nemesis to his death.
Poe mentions the jingling bells many times throughout the story. The last time he refers to Fortunato's bells comes very near the end.
No answer still. I thrust a torch through the remaining aperture and let it fall within. There came forth in return only a jingling of the bells.
This excerpt from the story's last paragraph should show Poe's intention. The reader cannot visualize Fortunato because he is completely walled up. Even Montresor, whose point of view has been the reader's point of view throughout the story, cannot see his victim. The jingling of bells behind the newly erected wall suggests that Fortunato is still alive and may remain in that horrible situation for a long time before he expires. The jingling bells have been telling Montresor and the reader the whereabouts of the victim in the pitch-darkness, and they serve as absolute proof of Fortunato's final fate. Poe cannot describe Fortunato in chains after the wall is completed because the author does not want to depart from Montresor's point of view; and Montresor cannot see what the scene is like behind the wall.
Montresor wanted, above all, satisfaction. The jingling bells in the last paragraph tell him that he has achieved his complete and perfect revenge. The bells place Fortunato in a site where he will never be found. After fifty years he will be nothing but another skeleton. When Montresor pens the final words, In pace requiescat, he is signifying that he has achieved full closure.