air castle vs daydreaming

air castle and daydreaming both are nouns.

Word	Noun	Adjective	Verb	Adverb
air castle	Yes	No	No	No
daydreaming	Yes	No	No	No

As nouns, air castle and daydreaming are synonyms defined as:

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

air castle (noun)	daydreaming (noun)
absentminded dreaming while awake	absentminded dreaming while awake