citadel vs acropolis

citadel and acropolis both are nouns.

Word	Noun	Adjective	Verb	Adverb
citadel	Yes	No	No	No
acropolis	Yes	No	No	No

As nouns, acropolis is a hyponym of citadel; that is, acropolis is a word with a more specific, narrower meaning than citadel:

citadel: a stronghold into which people could go for shelter during a battle
acropolis: the citadel in ancient Greek towns

Other hyponyms of citadel include kremlin.

citadel (noun)	acropolis (noun)
a stronghold into which people could go for shelter during a battle	the citadel in ancient Greek towns