The narrator of the story tells us that there are seven rooms, laid out from east to west, each of varying color, with windows painted to match the colors on the walls, floors, and ceiling: blue, purple, green, orange, white, violet, and, finally, black. However, the final black room has windows painted a "deep blood color." Further, in this room sits a large ebony (black) clock, and when this clock strikes the hour -- especially the midnight hour -- all the masqueraders pause and seem to fear something: even "the giddiest grew pale, and the more aged and sedate passed their hands over their brows as if in confused reverie or meditation." Clocks are often symbols of mortality, as is midnight, as are the colors black and red, especially a "blood" red. Poe layers this symbolism, it seems, to help us understand that this room represents death, and, since it is the westernmost room, the easternmost room, then, likely symbolizes birth (just as the sun's progress in the day, from sunrise to sunset, often symbolizes the span of a human life). The party-goers frequent these other rooms, but they prefer to stay out of the room of black and red, just as they fear the clock striking midnight, because they fear death. This fear, after all, is what has prompted them to quarantine themselves in the prince's abbey in the first place.
The rooms are laid out from East to West and they are done so as a metaphor for life. Many people use the sunrise and the sunset as a metaphor for life. The sun rises in the East the world lights up and so life begins. The sun shines brighter until midday and then the light begins to wan just like life. Finally the sun sets in the west and the world goes dark, just as when we die. The rooms in the abbey are also set up as such and the colors represent each stage of life that we pass through.