1. [n]  a dark cell (usually underground) where prisoners can be confined
2. [n]  the main tower within the walls of a medieval castle or fortress

Dungeon
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1. \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. -- Tyndale.
         Year after year he lay patiently in a dungeon. --
                                                  Macaulay.
   

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

Zork

different from the ordinary prison in being more severe as a place of punishment. Like the Roman inner prison (Acts 16:24), it consisted of a deep cell or cistern (Jer. 38:6). To be shut up in, a punishment common in Egypt (Gen. 39:20; 40:3; 41:10; 42:19). It is not mentioned, however, in the law of Moses as a mode of punishment. Under the later kings imprisonment was frequently used as a punishment (2 Chron. 16:10; Jer. 20:2; 32:2; 33:1; 37:15), and it was customary after the Exile (Matt. 11:2; Luke 3:20; Acts 5:18, 21; Matt. 18:30).