Poe is a master at creating suspense and total effect in all his pieces. "The Fall of the House of Usher" is no exception. From the very beginning when the narrator drops everything to go to his friend's aide, and he approaches the house which has "vacant eyes" for windows and deteriorating exterior covered with vines, we get the same overwhelming sense of rot and doom that the narrator detects. The house is dying, and it is symbolic of the house's connection with the twins who live inside (also dying) and share the name USHER. So right away, we get it: The Fall of the House of Usher is both literal and figurative. The house, the twins, and the legacy of the Usher family are in mortal danger. The house is equally disturbing on the inside--dark walls, large cracks, the windows are dark and surrounded by dark draperies--appearing like a prison--and the house seems to breathe and live on its own. Roderick expresses concern to the narrator that the house itself is one reason why he is so agitated and disturbed. Therefore, the setting definitely contributes to the overall effect of dread and gloom. In addition, the diction and vocabulary used are more sophisticated and perhaps a bit antiquated, giving the feeling that the story happened long ago--contributing to the mystery and overall effect of the tale.
The narrator tells the story, and he appears to be a rational being who is unwilling to believe that there is anything paranormal going on about the house. The appearance of Roderick's sister as a ghostlike individual--apparently buried alive and causing the unnatural knocking and haunting of the place--increases the potential for supernatural and strange effects on the reader. This, along with the supposition that the house itself is alive and connected to the twins is a considerable leap to make. However, with the death of both twins, the house begins to crumble and "fall" literally into the black lake the narrator mentions upon his arrival, concludes that there is a living being within the house which depends on the lives of the twins to exist. The narrator barely escapes with his life, and watches with disbelief as the house crumbles into nothingness.
Poe does that by adopting the elements of Gothic literature which are a) descriptive language that target specific feelings of horror, coldness, fear, and nostalgia (especially nostalgia) in the reader, b) the inclusion of the supernatural in the narrative, c) the use of nature as a form of creating atmosphere (cold climate, overcast, shadowy, dark, wet, humid), d) the debate of normalcy (usually Gothic literature would exaggerate a character, or a situation to stimulate anxiety in the reader, for instance, the man Usher, his sister's disease, the house), e) and the inclusion of death as the last resort.
The diction and combination of words are also a mechanism in Gothic literature as it appeals to the nostalgia from the reader, the use of proper, old English, the over-formation of words, etc. All those are typical mechanics of Gothic lit and House of Usher is a prime example of it.