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]