castle [OE] Castle goes back to Latin castellum, ‘little fort’ from castrum ‘fort’. To build castles in the air [M16th] is to have daydreams or unrealistic fantasies. It comes from a Latin phrase used by St Augustine, who became bishop of Hippo in North Africa in ad 396. Another version, originally a translation from medieval French, is to build castles in Spain [LME]. This country was probably chosen because it was a distant place where it would have been extremely unrealistic to build—most of it was under the rule of the Moors at the time the phrase was first used in French.