citadel vs bastion

citadel and bastion both are nouns.

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

As nouns, citadel and bastion are synonyms defined as:

citadel and bastion: a stronghold into which people could go for shelter during a battle

citadel (noun)	bastion (noun)
a stronghold into which people could go for shelter during a battle	projecting part of a rampart or other fortification
	a stronghold into which people could go for shelter during a battle
	a group that defends a principle