The Collaborative International Dictionary of English v.0.48:

Citadel \Cit"a*del\, n. [F. citadelle, It. citadella, di?. of
   citt? city, fr. L. civitas. See City.]
   A fortress in or near a fortified city, commanding the city
   and fortifications, and intended as a final point of defense.
   [1913 Webster]

   Syn. -- Stronghold. See Fortress.
      [1913 Webster]

WordNet (r) 3.0 (2006):

citadel
    n 1: a stronghold into which people could go for shelter during
         a battle [syn: bastion, citadel]

Moby Thesaurus II by Grady Ward, 1.0:

32 Moby Thesaurus words for "citadel":
   acropolis, bastion, beachhead, blockhouse, bridgehead, bunker,
   castle, donjon, fasthold, fastness, fort, fortress, garrison,
   garrison house, hold, keep, martello, martello tower, mote, motte,
   peel, peel tower, pillbox, post, rath, redoubt, safehold,
   strong point, stronghold, tower, tower of strength, ward