fastness vs citadel

fastness and citadel both are nouns.

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

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

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

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

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