stronghold vs bastion

stronghold and bastion both are nouns.

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

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

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

Other hyponyms of stronghold include citadel, blockhouse, hold, donjon, dungeon, keep, redoubt.

stronghold (noun)	bastion (noun)
a strongly fortified defensive structure	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