fastness vs donjon

fastness and donjon both are nouns.

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

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

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

fastness (noun)	donjon (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
the quality of being fixed in place as by some firm attachment