Castles in Spain/in the air

This expression means daydreaming or wasting time on pipe dreams. The expression was originally French from the 13th century and first appeared in Jean de Meun’s Roman de la Rose (c.1277) which was translated into English by Chaucer in the late 14th century, “Thou shalt make castels thane in Spayne, and dreme of joye, all but in vayne.” The expression was also used by Charles d’Orleans (1394-1465) in Rondeaux, “All by myself, wrapped in my thoughts, and building castles in Spain.” The expression building castles in Spain dates from the 15th century and building castles in the air dates from a century later. Why Spain? It appears to be a reference to the Moorish conquest and occupation of most of Spain from the 8th century onwards. For many centuries thereafter, building castles in Spain were viewed as fruitless exercises because the Moors kept knocking them down. It was not until the 15th century that the Spanish finally managed to expel the Moors from Spain.