How does Poe use the setting in the final scenes to transform the story's atmosphere? How do the descriptive details of the house strengthen the story?

Just as the house's scale and stability inspire the narrator with awe as well as fear, so does Usher's madness inspire as well as terrify him. That give-and-take, the dialectic between the beautiful and the horrifying, between amazement and dread, informs not just "The Fall of the House of Usher" but Poe's work in general. There is indeed a poetic quality to his writing, whether it be the use of the "Haunted Palace" as a metaphor for the mind--invaded by "evil things, in robes of sorrow"--or the description of the House of Usher as if it were a human face, with its "vacant eye-like windows." The Narrator describes, early in the story, "an unredeemed dreariness of thought which no goading of the imagination could torture into aught of the sublime." The infection the narrator increasingly felt becomes personified in the ultimate terrifying destruction of the house.