The phrase "My other castle" often implies a figurative meaning, suggesting there is a separate place or space where a person feels secure, safe, or like their own personal kingdom. This could be a literal building, like a family home, or a metaphorical space like a friend's house or a place where they feel most comfortable and free to be themselves. Link
The story of Mario who is constantly looking for his wife every week in another castle is a perfect example, he never knew his wife was planning her kidnap in other for her to spend time with the love of her life.
A castle is usually seen as a place of refuge, a place of comfort. So my other castle would reflect the same meaning as the main castle. Or perhaps the character is tired of the castle and now takes refuge in the other castle. But it is quite the privilege to have more than one castle.
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