E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
in fortification, a small strong fort, constructed either within the place fortified, or on the most inaccessible spot of its general outline; to give refuge for the garrison, that it may prolong the defence after the place has fallen, or to hold out for the best terms of capitulation. Citadels generally command the interior of the place and are useful, therefore, for overawing a population which might otherwise strive to shorten a siege. (French, citadelle; Italian, citadella, a little city.)