castle in Spain vs castle in the air

castle in Spain and castle in the air both are nouns.

Word	Noun	Adjective	Verb	Adverb
castle in Spain	Yes	No	No	No
castle in the air	Yes	No	No	No

As nouns, castle in Spain and castle in the air are synonyms defined as:

castle in Spain and castle in the air: absentminded dreaming while awake

Other synonyms of castle in Spain include air castle, daydream, daydreaming, oneirism, reverie, revery.

castle in Spain (noun)	castle in the air (noun)
absentminded dreaming while awake	absentminded dreaming while awake