stronghold vs citadel

stronghold and citadel both are nouns.

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

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

stronghold: a strongly fortified defensive structure
citadel: a stronghold into which people could go for shelter during a battle

Other hyponyms of stronghold include bastion, blockhouse, hold, donjon, dungeon, keep, redoubt.

stronghold (noun)	citadel (noun)
a strongly fortified defensive structure	a stronghold into which people could go for shelter during a battle