Prince Prospero's party is
In Edgar Allan Poe's "The Masque of the Red Death," Prince Prospero's party is, very significantly, a masquerade. For everything about this party is masked: The guests are only from the prince's social level--"a thousand hale and light-hearted friends--and the location is in, not his residential castle, but his "castellated abbeys," which are either monasteries or convents. Their resemblance to a castle is only in their castle-like towers.
So, not only are the "friends" disguised, but the location of the party is masked by being within an ostensibly humble, non-secular, and somewhat holy location. Even the rooms are masked as there is no "light of any kind emanating from lamp or candle within the suite of chambers." And, the rooms' windows are painted over, the drapes heavy velvet tapestries that "hung all over the ceiling and down the walls, falling upon the carpets. The outside of the abbey, there is a strong and lofty wall that "girdled it." This wall has gates of iron; thus, the revelers are concealed as well as protected by wall and fence in Prospero's effort to keep the Red Death away.
Well... this is a very open ended question with not much to let us know what you need.
I guess you could start by saying that this party is a masquerade ball (a masque). Prince Prospero is having this ball in his castle. He and a select group of his nobles and such have retreated inside the castle to try to avoid the Red Death -- the plague that is sweeping his kingdom.
The party is held in seven rooms of his castle. Each of these rooms has its own color theme and its own fantasy.