donjon
The fortified tower and main residence of a motte or early castle; a keep.
It was a fortress of no great size, consisting of a donjon, or large and high square tower, surrounded by buildings of inferior height, which were encircled by an inner court-yard.
Where it comes from
A variant of dungeon remodelled on its etymon, Old French donjon.
Adapted from the Wiktionary entry donjon (CC BY-SA 4.0).