castle in the air vs dream

castle in the air and dream both are nouns.

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

As nouns, dream is a hypernym of castle in the air; that is, dream is a word with a broader meaning than castle in the air:

Other hypernyms of castle in the air include dreaming.

castle in the air (noun)	dream (noun)
absentminded dreaming while awake	a series of mental images and emotions occurring during sleep
	imaginative thoughts indulged in while awake
	a fantastic but vain hope (from fantasies induced by the opium pipe)
	a state of mind characterized by abstraction and release from reality
	a cherished desire
	someone or something wonderful

castle in the air (verb)	dream (verb)
	have a daydream; indulge in a fantasy
	experience while sleeping