Why do Prince Prospero and his friends lock themselves in a secured castle?
In "The Masque of the Red Death," by Edgar Allan Poe, Prince Prospero is established as extremely disconnected, both physically and mentally, from the world around him. As the plague destroys the population around his castle, he remains "happy and dauntless," bringing in a thousand of his closest friends in order to weather the storm of death in mirth and merriment together. He seals off his castle from the outside world, stocking it with ample provisions, and believing it "folly to grieve or to think," as "[t]he external world would take care of itself."
Prospero believes his wealth separates him from the problems of the larger world around him, as this act of seclusion attempts to prove. The prince and his guests remain separated from the world, during which time they do nothing but engage in mirth and pleasure. As a means of celebrating their believed invulnerability, he throws a magnificent ball "toward the close of the fifth or sixth month of his seclusion, and while the pestilence raged most furiously abroad." It is at this time that the prince and his fellow revelers experience the harsh reality that nobody is safe from the plague. As the clock strikes midnight on the night of the masquerade ball, the Red Death makes its presence known, sweeping through the crowd of revelers and killing all of them, including the prince himself.
In "The Masque of Red Death" Prince Prospero and his friends lock themselves in a secured castle to avoid becoming in contact with "The Red Death", which is an epidemic that is killing everyone in town. The disease is described much like consumption, in that the last stages of it, blood will be everywhere, hence the name of the illness.