One of the things "The Fall of the House of Usher" is notable for is Roderick's recitation of a poem called "The Haunted Palace." The poem tells a story, setting out in lurid detail the literal and figurative fall of the House of Usher, an old noble family now in a state of permanent decline.
But things didn't always used to be this way. The poem's early stanzas give us a tantalizing glimpse into the House of Usher's glorious history. Then the house—that is to say, the physical building where the Ushers live—was a beautiful, stately palace ruled "in the monarch Thought's dominion."
The palace is used here by Poe as a symbol of the mind, which in the case of the Ushers was once healthy but which has since deteriorated sharply. Back in the good old days the house had "two luminous windows" providing wanderers in that "happy valley" with a sight of the "spirits moving musically." In other words, the Ushers's collective mind was harmonious, rational, and lucid, with a strong connection to the outside world.
Contrast this idyllic picture with the last stanza, when travelers in the valley, through the red-lit windows, can now only see strange, vast forms moving chaotically to a discordant melody. The House of Usher's glory days are now well and truly over; now it has descended so far into madness that the only people who show up at this decaying, ramshackle place are frightened travelers and a "hideous throng" laughing maniacally. How the mighty have fallen!