What do people "build" when they make unrealistic plans for the future?

Building castles in the air means entertaining daydreams that will never come to pass.

The idiom "building castles in the air" first appeared in the 1500s, as a translation from the French. However, the original, literal translation of the French idiom was "building castles in Spain".

The first translation of the phrase into English came from a work of French poetry known as "Roman de la Rose", translated in the mid-1300s: “Thou shalt make castles than in Spaine, And dreame of joy, all but in vaine …”

This poem is a treatise on desire, romantic ideas and romantic love. The idiom "building castles in Spain" was well known at this time to mean something that is impossible to accomplish. Spain, held by the Moors, was an impenetrable force to the French, and not a place one could conquer and build a castle in.

By the 1500s the reference to building castles in Spain was no longer understood, and fresh translations of the "Roman de la Rose" used the idiom "building castles in the air", instead.