bastion vs fastness

bastion and fastness both are nouns.

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

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

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

Other hypernyms of bastion include stronghold.

bastion (noun)	fastness (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 rate (usually rapid) at which something happens
a group that defends a principle	the quality of being fixed in place as by some firm attachment