fastness vs bastion

fastness and bastion both are nouns.

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

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

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

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

fastness (noun)	bastion (noun)
a strongly fortified defensive structure	projecting part of a rampart or other fortification
a rate (usually rapid) at which something happens	a stronghold into which people could go for shelter during a battle
the quality of being fixed in place as by some firm attachment	a group that defends a principle