dungeon vs fastness

dungeon and fastness both are nouns.

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

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

Other hypernyms of dungeon include stronghold.

dungeon (noun)	fastness (noun)
the main tower within the walls of a medieval castle or fortress	a strongly fortified defensive structure
a dark cell (usually underground) where prisoners can be confined	a rate (usually rapid) at which something happens
	the quality of being fixed in place as by some firm attachment