fastness vs dungeon

fastness and dungeon both are nouns.

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

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

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

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