citadel vs fastness

citadel and fastness both are nouns.

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

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

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

Other hypernyms of citadel include stronghold.

citadel (noun)	fastness (noun)
a stronghold into which people could go for shelter during a battle	a strongly fortified defensive structure
	a rate (usually rapid) at which something happens
	the quality of being fixed in place as by some firm attachment