donjon vs dungeon

donjon and dungeon both are nouns.

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

As nouns, donjon and dungeon are synonyms defined as:

donjon and dungeon: the main tower within the walls of a medieval castle or fortress

Other synonyms of donjon include keep.

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