bastion vs stronghold

bastion and stronghold both are nouns.

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

As nouns, stronghold is a hypernym of bastion; that is, stronghold is a word with a broader meaning than bastion:

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

Other hypernyms of bastion include fastness.

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