Castle's construction was started as soon as Westley's mother realized she was pregnant, so Castle was built and enchanted within a year so as to be ready for him as soon as he was born. She is only a castle due to the fact Westley is a major weather worker; other sentient structures tend to be things like cottages for smaller nature workers.

Castle can change her location at will, depending on where Westley has to be to influence the weather. She can be on land, underwater, floating in the sky, inside a volcano.... Anywhere. In the more hostile environments, a protective dome forms over her, starting at the wall around the grounds.

Due to Westley's mother dying giving birth to him, Castle has been the main influence on Westley's life, and has been a cross between a mother and sister to him, despite the fact she's only a year older. She provides everything he needs, causing anything from food to clothes to appear as necessary. She, like other sentient structures, has a heart room, with a crystal in the centre where she could grow an avatar, something that takes five years. Her avatar is fairly strong, due to the fact her actual body is made of stone and wood. Being a creation of magic, she has some control over magic, but she rarely uses it.

If her avatar is destroyed, she can grow another one so long as her heart room is not compromised.