daydreaming vs air castle

daydreaming and air castle both are nouns.

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

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

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

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