Drum tower

Drum Tower - A large, circular, low, squat tower built into a wall.
Drystone - Unmortared masonry.
Dungeon - The jail, usually found in one of the towers.

Drum Tower: a round tower built into a wall
Dungeon: the jail, usually found in one of the towers
Enceinte: an enclosing wall, usually exterior, of a fortified place ...

Drum tower: A squat round tower which was used as a flanking tower for curtain walls, gatehouses and barbicans. See flanking tower.

See also: Postern, Oubliette, Turning bridge, Moat, Loophole