Idiom: Extravagant hopes and plans that will never be carried out.
http://dictionary.reference.com/search?q=castl es+in+the+air&r=66
Castles in the air : a fanciful or impractical notion or hope; daydream. Also called air castle, cas'tle in Spain'.
http://www.infoplease.com/ipd/A0364388.html
By reading the characteristic features of any man's castles in the air you can make a shrewd guess as to his underlying desires which are frustrated - John Dewey
Neurotics build castles in the air, psychotics live in them. My mother cleans them - Rita Rudner
Although a new or useful object or method may be developed to fulfill a specific purpose, the original idea may never be fully realised as a working invention, perhaps because the concept is in some way unrealistic or impractical.
A "castle in the air" or a "pie in the sky" (or "castles in Spain") may refer to a creative idea which does not reach fruition due to practical considerations. The history of invention is full of such castles, because inventions are not necessarily invented in the order that is most useful; for example, the design of the parachute was worked out before the invention of powered flight.
http://en.wikipedia.org/wiki/Castles_in_the_ai r