The story as a suspenseful mood to it. We start when Montresor is talking about the many injuries Fortunato left him and that he will take his revenge. The suspense is when we wait for Montresor to take his revenge and we don't know what it will be.
The story had a morbid tone because of the catacombs that were dark, a little mysterious and with bones around them. Also the way he took his revenge was also morbid because the author could of made a simpler revenge for the story.
Revenge is a dish best served cold. Montresor carefully planned his revenge on Fortunato, and he slowly executed it making sure there were no mistakes. In my opinion revenge is a dish best served cold means that waiting properly to do something is better in the end than acting in the moment without thinking.