Dun"geon (?), n. [OE. donjoun highest tower of a castle, tower, prison, F. donjon tower or platform in the midst of a castle, turret, or closet on the top of a house, a keep of a castle, LL. domnio, the same word as LL. dominus lord. See Dame, Don, and cf. Dominion, Domain, Demesne, Danger, Donjon.] A close, dark prison, common&?;, under ground, as if the lower apartments of the donjon or keep of a castle, these being used as prisons.

Down with him even into the deep dungeon.

Year after year he lay patiently in a dungeon.

Dun"geon, v. t. To shut up in a dungeon. Bp. Hall.

• (originally) the main tower of a motte or castle; a keep or donjon
  
• An underground prison or vault, typically built underneath a castle