citadel vs kremlin

citadel and kremlin both are nouns.

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

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

citadel: a stronghold into which people could go for shelter during a battle
kremlin: citadel of a Russian town

Other hyponyms of citadel include acropolis.

citadel (noun)	kremlin (noun)
a stronghold into which people could go for shelter during a battle	citadel of a Russian town