Why do Prince Prospero's courtiers weld the gates shut?
When the prince and his courtiers sealed themselves in the abbey, they were trying their best to avoid the terrible disease that was ravaging their land, so they were obviously willing to go to extremes in order to protect themselves from dying. They welded the gates of the abbey shut for two reasons: to keep things both out and in. They did not want anyone coming into the abbey who might potentially be infected because then their attempts to stay healthy would be futile; the disease would easily spread through the large crowd and they would all be dead, probably within just a single day. They also did not want anyone to change their minds about staying safe within the abbey walls and leave because then that person would most likely die; being locked up in a single place for an indefinite amount of time is a surefire way to get cabin fever, and the prince and his courtiers did not want to die simply because they got bored one day and decided to risk venturing out into the infected world.