(n.) a daydreamer, a fantasist
If you’re a castle-hunter, then you’re the kind of person who likes to get lost in your own thoughts or dreams, and indulge in wild fantasies about who or what you might become.
That word, alongside the synonymous castle-builder, alludes to the more familiar expression to build castles in the sky, which has been used to mean ‘to imagine wildly unlikely outcomes’, or ‘to form unrealistic plans’, since the late Middle English period.
Before then, English speakers used the French expression faire les châteaux en Espagne, ‘to build castles in Spain’, to imply precisely the same improbably outcome—and before then, French speakers as far back as the 1300s were ‘building castles in Albania’ (faire les châteaux en Albania), or ‘in Asia’ (en Asie).
No matter the form of this expression, the implication is the same: the daydreamer is imagining a life of opulence, success and high standing in some improbable, impossible, or far-flung location where they have no real influence or standing. It’s for good reason that French speakers appear to have singled out Albania, Asia and Spain, as they would have seemed as far removed from European Christendom as could be imagined in the Middle Ages (much of Spain at the time being under Moorish control).