What does the castle and all its barricades symbolize in "The Masque of the Red Death"?

In Edgar Allan Poe's "The Masque of the Red Death," after
Prince Prospero's "dominions were half depopulated" by the plague that was spreading it
red stained death upon the kingdom, he decides to beckon the knights and his friends to
one of his "castellated abbeys."  In the medieval ages, abbeys were built for monks, but
often they were used as bulwarks against enemies.  So, Prospero gathers people from his
kingdom in this abbey in an effort to prevent the Red Death from entering and killing
any more of his subjects.  He hopes that the abbey, a fortress with its "lofty wall with
gates or iron," will prevent any unwanted guest from entering.  Having welded shut the
iron gates and sealed themselves inside the "lofty wall," the prince and his guests
have, they feel, "defiance to contagion," and they leave the external world to "take
care of itself."  Thus, the barricades and walls of the catellated abbey represent man's
efforts to fight an abstract force with material objects, demonstrating how people react
to their mortality.  Sadly, no physical object or force can prevent death from
entering.