From WordNet (r) 3.0 (2006) [wn]:
citadel
n 1: a stronghold into which people could go for shelter during
a battle [syn: bastion, citadel]
From The Collaborative International Dictionary of English v.0.48 [gcide]:
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]