Citadel, n.
Etymology: [F. citadelle, It. citadella, dicitt 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. Syn. - Stronghold. See Fortress.
Shades of meaning
Fernald has no entry of its own for citadel; he lists it under fortification.
Where he writes about it:
A citadel is a fortification within a city, or the fortified inner part of a city or fortress, within which a garrison may be placed to overawe the citizens, or to which the defenders may retire if the outer works are captured; the medieval castle was the fortified residence of a king or baron. — from fortification