1. A dark, often underground chamber or cell used to confine prisoners, especially beneath a castle: In the adventure story, the dungeon was different from the ordinary prison in being more severe as a place of punishment.
2. Archaic term, the secure main tower of a castle; The dungeon, a keep or donjon, was very tall and surrounded by a large ditch with water.

This word comes ultimately from Latin dominus "lord, master"

This was derived from dominium "property" (source of English dominion), that in post-classical times became domino or domnio, meaning "lord's tower".

In Old French this became donjon, the term for a "castle keep", and eventually, by extension, a "secure (underground) cell". The form dungeon developed the specialized sense of strong closed cell, underground place of confinement; based on the French donjon (large tower of a castle).