E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Castle in the Air.
A splendid edifice, but one which has no existence. In fairy tales we often have these castles built at a word, and vanishing as soon, like that built for Aladdin by the Genius of the Lamp. These air-castles are called by the French Châteaux d Espagne, because Spain has no châteaux. We also find the expression Châteaux en Asie for a similar reason. (See CHATEAUX.)