noun
a large building or group of buildings fortified against attack with thick walls, battlements, towers, and in many cases a moat
preposition
a word governing, and usually preceding, a noun or pronoun and expressing a relation to another word or element in the clause
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The idea of starting a business without a solid plan is like building a castle in the air.
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Thinking about becoming a millionaire overnight is just like building a castle in the air.
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Dreaming about winning the lottery is like building a castle in the air.
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His plans for a lavish vacation seemed like a castle in the air, considering his limited budget.