In reference to the house, "No outlet was observed in any portion of its vast extent, and no torch, or other artificial source of light was discernible."
In reference to Usher, "To an anomalous species of terror I found him a bounden slave."
In reference to Madeline, "The disease which had thus entombed the lady in the maturity of youth."
Poe of course shows that Madeline was also trapped in her coffin.
The whole story seems to represent an entrapment of the human mind or soul in darkness. With each of these instances a character is bound to something that was not physically removable. This could signify a malady within the narrator that distorted his perception of the people he was with, or it could be a reality Poe suffered himself, if only in his mind.
Ad augusta per angusta,
Will Drake