What is the message of "The Masque of the Red Death"?

The message of Poe's story is that no artifice can prevent the inevitability of Fate.

While death inevitably comes to all mortals, there are many who seek to defy this fate. As one of the many who do, Prince Prospero takes measures to stop the approach of the Red Death by creating a bulwark against its advances. In order to arrest the linear movement of time, Prospero creates an intricate pattern of geometric space, a distinct departure from the traditional arrangement in palaces that is linear so that when walls are folded "the whole extent is scarcely impeded." Prospero's arrangement does not allow for any linear view; instead, the rooms are positioned irregularly so that no one can see farther than what is in that room. There is an intricate geometric pattern created by "a sharp turn at every twenty or thirty yards, and at each turn, a novel effect." Then, in the middle of each wall, there is a Gothic window that looked upon only a closed corridor. In addition, there is no light in any of the rooms; light emanates only from these corridors outside the rooms. With such an arrangement, Prospero hopes to further fortify his guests from the Red Death's ability to pass from room to room. 

Despite all these "bold and fiery" plans against the Red Death and the "multitude of dreams" created by the glare and glitter, arabesque figures, and bizarre and "delirious fancies," Time yet imposes itself as a huge ebony clock, whose pendulum swings with a monotonous and heavy sound. When it strikes the hour, its "brazen lungs" cause the musicians to arrest their performance, and all the guests grow pale, trembling in their apprehension. And, when the final hour bellows from this clock, there appears the horrific figure dressed in the "habiliments of the grave." Despite Prospero's "the immutable realm of art," nothing can prevent the advance of time and Death that makes its way "uninterruptedly" through all the rooms, killing everyone.