air castle vs castle in Spain

air castle and castle in Spain both are nouns.

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

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

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

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