castle in the air vs castle in Spain

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

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

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

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

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

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