Origin, Meaning of the word 'Donjon'
The meaning and origin of the castle keep explain this unusual word! The word 'donjon' was originally used to describe the castle keep (the tower which was built on top of the motte). The origins of the word 'Donjon' was derived from a Latin word meaning 'Dominating Point'. Later the word 'Donjon' was changed to 'Keep'.
The term 'Donjon' was not lost - it was a derivative for the word 'Dungeon' - the Nobles moved from the towers to more easily accessible living quarters and important prisoners were held in the 'donjons'. As time passed these towers for prisoners ceased to be built and prisons were built below ground level - the dungeon!