A castle in the air is a wishful or imaginary project, a pipe dream, an idle speculation. In French they say a castle in Spain for the same idea, and that expression is sometimes used in English too.
In the singular, Castle in the Air is a book by Diana Wynne Jones; and is the eighteenth chapter of Norton Juster's The Phantom Tollbooth, being the castle in which the Princesses Rhyme and Reason are imprisoned, and which is attached to the Mountains of Ignorance by a ricketty staircase. But enough of singulars; they should have their own node.
'Castles in the Air' is one of Don McLean's best songs. It featured on his debut album Tapestry in 1970, and was released as a single in 1972, the double A-side with 'Vincent', the tale of van Gogh's starry, starry night. A slightly different version of 'Castles in the Air' appeared on his 1982 album Believers, and has naturally been included in some compilations, such as And I Love You So (EMI, 1989).
It's a fast-paced tale of despair at the worldliness of a woman. The singer is tired of her castles in the air, then rather incongruously says that he is a dreamer. But her world, her castles, are 'devoid of all romance'; while his are outdoorsy things, mountains and forests, and settling down into his dream-world there.
The singer is weak; he even says so. The verses vacillate quickly between a kind of contempt for her, as if he's taking charge and casting her off, and a fearful inability to face her. He's asking his friend to do the hard part. The song opens, suddenly, with an upbeat request to do just that ('And if she asks you why'): we get the feeling he half expects the wretched woman to come crawling and weeping when she sees what she's losing.
But the reality is he knows she won't come back, won't care, won't miss him amid her 'cocktail generation', and he's trying to salvage a little dignity by trying to get out on what's left of his own terms:
Save me from all the trouble and the pain
I know I'm weak, but I can't face that girl again
He gets lyrical when he's talking about the wind and the sunlight. Wouldn't it be nice to free, and on his own, and free to find a country woman for his wife! And the tone switches to and fro, between imaginative hope and hurt, exhausted wretchedness. But the castles in the air are largely his own. A familiar, sad tale of self-deception in unfulfilled love.
Quoted portions are about 10% of the original lyrics and should be fair use.