A good example of symbolism in "The Masque of the Red Death" is the imperial suite in which Prince Prospero and his guest hold their party. Instead of being a long corridor of rooms, connected by doors so that the entire length can be made visible, the suite is formed of rooms that connect around sharp corners, and each room is styled with a different color:
To the right and left, in the middle of each wall, a tall and narrow Gothic window looked out upon a closed corridor which pursued the windings of the suite. These windows were of stained glass whose color varied in accordance with the prevailing hue of the decorations of the chamber into which it opened.
(Poe, "The Masque of the Red Death," xroads.viginia.edu)
Because of the prevailing theme of impending and inevitable death, the rooms symbolize various stages of the human life. Each room is colored differently, because each part of a human life is different; the rooms cannot be seen from one another, because the future is hidden and the past cannot be changed. As the Masque itself passes through each of the rooms, it is unhindered; the last room, where the Masque is revealed, is black with red glass, the colors of death and the plague itself. As each guest dies in their respective room, the random and senseless nature of death is seen; people often die in childhood, or in accidents, and so the rooms represent both the length and finite nature of life, and the pain of dying before one's time.
The clock represents time "ticking" away. It shows that no matter where you go or what money you have time never stops.
The rooms represent life. You cannot see in the rooms ahead of you just like you cannot see your future in life. You must go ahead and live it to see. Just like you must walk into the room to see what it holds.