Definition of "castle in the air" []

• A visionary project or scheme; a day-dream; an idle fancy; a pipe dream; any plan, desire, or idea that is unlikely to be ever realized; a near impossibility. (noun)

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• "One of the mighty geological contortions for which the southern Tien Shan are famous had dislocated the strata through ninety degrees into the vertical, creating a castle in the air unassailable except on foot, cradling a walnut forest in its plateau."