• (RP) IPA: /ˈkɑːs(ə)l ɪn ðiː ˈɛə/
• (GA) IPA: /ˈkæs(ə)l ɪn ði ˈɛəɹ/

castle in the air

1. (idiomatic) A desire#Noun|desire, idea, or plan#Noun|plan that is unlikely to ever be realized; a visionary project#Noun|project or scheme#Noun|scheme; a daydream#Noun|daydream, an idle fancy#Noun|fancy, a near impossibility. [from mid 16th c.]

   Synonyms: air castle, castle in Spain, castle in the skies, eggs in moonshine, jam tomorrow, pie in the sky, pipe dream

   • 1624, Democritus Junior [pseudonym; Robert Burton], “Of the Force of Imagination”, in The Anatomy of Melancholy: […], 2nd edition, Oxford, Oxfordshire: Printed by John Lichfield and James Short, for Henry Cripps, OCLC 54573970 ↗, partition 1, section 2, member 3, subsection 2, page 81 ↗:

     Many times ſuch men when they come to themſelves, tell ſtrange things of Heauen and Hell, what viſions they haue ſeene; […] The like effects almost are to bee ſeene in ſuch as are awake: How many Chimæras, Anticks, golden mountaines, and Caſtles in the Aire doe they build vnto themſelves?

   • 1822, [Walter Scott], chapter III, in Peveril of the Peak. [...] In Four Volumes, volume III, Edinburgh: Printed for Archibald Constable and Co.; London: Hurst, Robinson, and Co., OCLC 2392685 ↗, page 70 ↗:

     Her unlimited devotion for "the family," readily induced the old lady to acquiesce in his proposal, though not without a gentle sigh over the ruins of a castle in the air, which was founded on the well-saved purse of Mistress Deborah Debbitch.

   • 1847 January – 1848 July​, William Makepeace Thackeray, “Rebecca is in Presence of the Enemy”, in Vanity Fair. A Novel without a Hero, London: Bradbury and Evans, […], published 1848, OCLC 3174108 ↗, page 18 ↗:

     She had a vivid imagination; […] and it is a fact, that while she was dressing for dinner, and after she had asked Amelia whether her brother was very rich, she had built for herself a most magnificent castle in the air, of which she was mistress, with a husband somewhere in the background (she had not seen him as yet, and his figure would not therefore be very distinct); {{...}

   • 1854 August 8, Henry D[avid] Thoreau, “Conclusion”, in Walden; or, Life in the Woods, Boston, Mass.: Ticknor and Fields, OCLC 4103827 ↗, page 346 ↗:

     If you have built castles in the air, your work need not be lost; that is where they should be. Now put the foundations under them.

• French: châteaux en Espagne
• German: Luftschloss
• Portuguese: castelo de vento, castelo no ar
• Russian: возду́шный за́мок
• Spanish: castillos en el aire, castillos de naipes