dungeon vs donjon

dungeon and donjon both are nouns.

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

As nouns, dungeon and donjon are synonyms defined as:

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

Other synonyms of dungeon include keep.

dungeon (noun)	donjon (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