citadel vs stronghold

citadel and stronghold both are nouns.

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

As nouns, stronghold is a hypernym of citadel; that is, stronghold is a word with a broader meaning than citadel:

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

Other hypernyms of citadel include fastness.

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