Why does Prospero lock himself and his friends in his abbey in "The Masque of the Red Death"?

Prince Prospero sequesters himself and his guests in his abbey for protection from the Red Death.

Poe's "Masque of the Red Death" is often read as an allegorical tale of man's fearful attempt to defy his mortality. The seven rooms represent the seven stages of man from infancy to very old age: The east room that is blue represents birth and the last room in black represents death. Thus, the rooms symbolize the temporal nature of life.
The prince and the members of his court hope to escape the plague that spreads through the area. By engaging in revelry they hope to distract themselves from the plague that is a threatening reality. But, with the ominous sounding of midnight from "the brazen lungs" of the gigantic ebony clock, the guests shudder as they contemplate their own mortality. Then, when a stranger, "shrouded from head to foot in the habiliments of the grave" that are dabbled with blood appears in the abbey, the guests shudder in horror of this deadly specter.
Enraged at the intrusion of this specter, Prince Prospero, who stands in the blue room, traverses the other rooms and confronts this "pursurer." A sharp cry is heard as Prospero drops his dagger, collapsing to the floor of the dark room. The prince has met the Red Death.