Suppose that Montresor’s wine cellar was not in the catacombs. How would the plot change?
Montresor states that Fortunato is wearing a "tight-fitting" jester's costume when he encounters him carousing on the street. Montresor has a rapier concealed under his cloak. He reveals this fact for the first time when Fortunato begins screaming for help.
A succession of loud and shrill screams, bursting suddenly from the throat of the chained form, seemed to thrust me violently back. For a brief moment I hesitated, I trembled. Unsheathing my rapier, I began to grope with it about the recess...
Montresor's main problem is to get Fortunato to come to his palazzo and down into the wine cellar without being recognized by anyone on the streets as Fortunato's companion. Montresor is armed; Fortunato must be unarmed because there would be no place to conceal a weapon in a tight-fitting costume. Montresor could simply murder Fortunato at the foot of the steps. Then he would have to bury him somewhere under the palazzo. This sounds like a hackneyed plot. How many men in stories and novels have murdered people and buried them in their cellars?
The trip through the catacombs makes Poe's story most interesting. Chaining Fortunato inside the narrow niche and building a wall to conceal him also add interest. Without such details the plot would lack some of its best effects, such as Fortunato's screaming for help and pleading for mercy, and the story would probably not have been remembered as long as it has.
I think that the biggest change to the plot if the story did not take place in the catacombs would be Montresor's method of murder. For instance, say Montresor took Fortunato to his house instead. There would be many different factors and far more risks involved in murdering someone in a house rather than below ground in the catacombs. The nice thing about the catacombs is that it is secluded, so no one will be around to interrupt Montresor's plan; no one will see or hear Montresor and Fortunato, so he is far more likely to get away with the murder. However, if Montresor had murdered Fortunato in his house, he would not have been able to seal Fortunato inside a recess in a wall because someone would most likely hear Fortunato screaming for help or notice the new wall. So, if Montresor were to have murdered Fortunato elsewhere, it is more likely that he would be caught, and then this story would start like Poe's The Black Cat, which starts with the narrator waiting to be executed, which we later find out is happening because he murdered his wife.