A large house or mansion where a young person (not yet moved out) lives with their parents. If someone lives in a "starter castle", they are usually from a wealthy family and are privileged. This "starter castle" is just a little beginner mansion for this young person who will undoubtedly be wealthy later in life.
Johnny: "Sam, did you see how big Sarah's house is?!"
Sam: "I know, right? Total starter castle."
by Acorn Man June 23, 2015
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