DONJON

keep, donjon, dungeon

(noun) the main tower within the walls of a medieval castle or fortress

Source: WordNet® 3.1

Etymology

Noun

donjon (plural donjons)

The fortified tower of a motte or early castle; a keep.

Source: Wiktionary

Don"jon, n. Etym: [See Dungeon.]

Definition: The chief tower, also called the keep; a massive tower in ancient castles, forming the strongest part of the fortifications. See Illust. of Castle.

Source: Webster’s Unabridged Dictionary 1913 Edition