stronghold vs dungeon

stronghold and dungeon both are nouns.

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

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

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

stronghold (noun)	dungeon (noun)
a strongly fortified defensive structure	the main tower within the walls of a medieval castle or fortress
	a dark cell (usually underground) where prisoners can be confined