stronghold vs donjon

stronghold and donjon both are nouns.

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

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

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

stronghold (noun)	donjon (noun)
a strongly fortified defensive structure	the main tower within the walls of a medieval castle or fortress