Usage: A fortress is constructed for military purposes only, and is permanently garrisoned; a fortification is built to defend harbors, cities, etc.; a castle is a fortress of early times which was ordinarily a palatial dwelling; a citadel is the stronghold of a fortress or city, etc.
Etymology: F. forteresse, OF. forteresce fortelesce, LL. foralitia, fr. L. fortis strong. See Fort, and cf. Fortalice