casino vs gambling den

casino and gambling den both are nouns.

Word	Noun	Adjective	Verb	Adverb
casino	Yes	No	No	No
gambling den	Yes	No	No	No

As nouns, gambling den is a hypernym of casino; that is, gambling den is a word with a broader meaning than casino:

casino: a public building for gambling and entertainment
gambling den: a public building in which a variety of games of chance can be played (operated as a business)

Other hypernyms of casino include gambling hell, gambling house, gaming house.

casino (noun)	gambling den (noun)
a card game in which cards face up on the table are taken with eligible cards in the hand	a public building in which a variety of games of chance can be played (operated as a business)
a public building for gambling and entertainment