You have asked more than one question and thus transgressed enotes regulations. I have therefore edited this question to make it more general about the story as a whole. Remember you are only allowed to ask ONE question per day in each group.
Poe was a master of writing stories of horror and revenge, and as a "Dark Romantic" the focus of so much of his work is the dark side of humanity - the capacity of us all to commit heinous acts. What is key to note about his work is how he uses masterfully the first person narrator to reveal deeply disturbed psyches and characters who often are unreliable narrators - in that we as readers can see that often there is more going on in their account than their words at first indicate.
This short story is of no exception - consider how it begins:
The thousand injuries of Fortunato I had borne as best I could; but when he ventured upon insult, I vowed revenge.
As the story progresses and we see in particular the friendly way in which Fortunato responds to the narrator, we begin to doubt the veracity of the narrator's comments - would Fortunato really entrust himself if he had insulted the narrator?
As the story moves both its characters and us as readers to the labyrinthine catacombs and underground darkness of Italy, we come to realise that we are being exposed to the darkness, or the "underground" emotions and feelings of the narrator. The setting therefore is a wholly appropriate place for the narrator to gain his terrible revenge - sealing a man into a room and leaving him to die a slow and terrible death. As we venture down into the catacombs, we go on a journey into the darkness and horror of the narrator's innermost desires, and thus we are shocked and terrified just as Fortunato is by what is revealed. Consider the following passage:
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. My heart grew sick - on account of the dampness of the catacombs.
The way in which the narrator describes the completion of his task in such a matter-of-fact way makes his actions all the more chilling, as does his disavowal that the "sickness" of his heart had anything to do with his act.
Hope this helps give you an idea of the story as a whole and comments that you can seize on and develop in your work. It is a great story - vintage Poe, and worthy of enjoyment!