The story
The first term dates from the late 1500s. A variant, castles in Spain (or châteaux en Espagne), was recorded in the Roman de la Rose in the 13th century and translated into English around 1365.
Etymology from Wiktionary, CC BY-SA 4.0.
build castles in the air
Today it means: To have any desire, idea, or plan that is unlikely to be realized; to imagine visionary projects or schemes; to have an idle fancy or a pipe dream; to daydream. Full definition →