The idea of being something else, more than someone else. A metaphor, a concept, to be a planet interacting with other planets, a sheep following the leading pigs, a house of emotions, where your thoughts speak their minds. The idea of being a castle. Interesting.
To be a castle, as a human being, means to have big walls, to keep people away. It means to have a big gate, to have control over who gets in. It means to have a king or lord, leading your people (character leading thoughts, emotions). It means to expand territory and create bonds with other castles.