Without doubt, the setting of Poe's Gothic masterpiece contributes greatly to the atmosphere of isolation, mystery, shadows, decay, and preternatural experiences. That the house is representative of the dying Usher family is indicated in this passage:
...about the whole mansion and domain there hung an atmosphere peculiar to themselves and their immediate vicinity—an atmosphere which had no affinity with the air of heaven, but which had reeked up from the decayed trees, and the grey wall, and the silent tarn.
A family in decline, the Ushers as the remaining descendants are like the mansion that is declining in decay--the narrator speaks of the Ushers as having "molded the[ir] destinities"; they, too, are mysterious, isolated, melancholy, and dying. Both the mansion and the Ushers themselves create in the narrator "a sickening of the heart, a dreariness," and a terror. When the narrator approaches the Usher mansion and as he flees while "the mighty walls rush asunder," it is with these same feelings of terror that he later watches Roderick descend into insanity and be killed by his cataleptic sister Madeline.
When the narrator first sees Madeline, he describes his reaction, a reaction not dissimilar to that of his sighting of the Usher mansion in which he describes his reactions as "a sickening of the heart," "a dreariness of thought" and an unnerving: "I regarded her with utter astonishment not unmingled with dread...."
Much like the wasting away and preternatural experiences of Madeline and her "similitude" with her brother, the Usher mansion exhibits bizarre sympathies. For example, the deterioration of both Madeline and the house are similar; the door to the vault makes a sharp grating sound similar to the "low moaning cry" with which she falls upon Roderick in her "death agony."
When the narrator first arrives, he senses a atmosphere around the mansion that seems connected with decay and disease. These conditions of decay and disease certainly apply to the Ushers themselves. For instance, Roderick suffers from "nervous agitation," bodily illness, and a "mental disorder which oppressed him." This disorder heightens all his senses to the point that he is tortured by sounds. He paints scenes that are supernatural and horrific, words that can also describe the eventual tearing asunder of the structure of the mansion. Like Roderick, the Usher mansion seems to fall victim to decay. Symbolic of Roderick's break with reality, there is a fissure in the wall of the mansion, a fissure that eventually widens and effects the end of the house, much as Roderick's death comes as he is victimized by his sister's cadaverous attack.