What is so fascinating about this tale is the rich, sensuous detail that Poe adds to the description of this party that Prospero holds, in spite of the death that is raging outside. In particular there are two details that greatly add to the meaning of the story, and they are the rooms and the clock. Notice how the rooms are described:
That at the eastern extremity was hung, for example, in blue--and vividly blue were its windows. The second chamber was purple in its ornaments and tapestries, and here the panes were purple. The third was green throughout, and so were the casements. The fourth was furnished and lighted with orange--the fifth with white--the sixth with violet. The seventh apartment was closely shrouded in black velvet tapestries that hung all over the ceiling and down the walls, falling in heavy folds upon a carpet of the same material and hue. But in this chamber only, the colour of the windows failed to correspond with the decorations. The panes were scarlet--a deep blood colour.
It is important to be aware of the symbolic significance of these rooms, both in terms of their colours and their positionings. Firstly, the fact that the rooms are laid out from east to west matches the course of the sun, that rises from the east and sets in the west. The colours as well seem to indicate the passing of time and aging, with black of course representing the final death of humans, which corresponds with its position in the west, where the sun sets. It is highly significant therefore that Prospero pursues the mummer or the figure disguised as the Red Death all the way through each room to the black room, which is where he dies. The message that these details help support is therefore reinforcing that death is inevitable and that it cannot be avoided, no matter how rich or powerful you are. The clock of course supports this message to, being a symbol of time passing and the impossibility of making time stand still.