The abbey to which Prince Prospero invites his guests for a secluded and highly decadent retreat while the common people die horrific deaths outside his castle walls is described aptly by Poe early in his story:
"This was an extensive and magnificent structure, the creation of the prince's own eccentric yet august taste."
Poe is suggesting that the abbey represents Prince Prospero's own personality -- a personality lacking in restraint and greatly out of touch with the world outside his castle gates. Once again, as Poe describes the prince's plans for the use of the abbey as a refuge from the plague spreading across the land:
"The abbey was amply provisioned. With such precautions the courtiers might bid defiance to contagion. The external world could take care of itself...All these and security were within. Without was the 'Red Death'."
The abbey also represents, if the repeated use of the colors red and black are an indication, death itself. The seven rooms within the abbey are described as being black and red, two colors that often represent death in literature. Black is commonly associated with death, and red references the "Red Plague" decimating the countryside.
Given the short length of Poe's story, that so much time is spent describing the abbey surely indicates its symbolic importance in the extent to which it is intended to reflect "the duke's love of the bizarre" and the pale of death hanging over the proceedings.
Another feature of the abbey as described by the author was the presence of a grandfather clock, with its swinging pendulum -- a central motif of another of Poe's macabre stories, "The Pit and the Pendulum." The use of time as indicated by this clock builds tension, as it's chimes are loud enough to bring all the merriment to a momentary standstill, and its black color once again suggestive of death.
Of particular note is the following description of the bridal chamber within the abbey:
"The room lay in a high turret of the castellated abbey, was pentagonal in shape, and of capacious size. Occupying the whole southern face of the pentagon was the sole window..."
Given Poe's intense interest in the occult, the pentagonal shape of the bridal chamber cannot be a matter of chance. Followers of the occult recognize the pentagon as the center of pentagram, and associate it with Satan. It is reasonable to conclude, therefore, that the abbey represents first and foremost evil.