air castle vs castle in the air

air castle and castle in the air both are nouns.

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

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

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

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