Fortress, n.; pl. Fortresses.
Etymology: [F. forteresse, OF. forteresce, fortelesce, LL. foralitia, fr. L. fortis strong. See Fort, and cf. Fortalice.]
A fortified place; a large and permanent fortification, sometimes including a town; a fort; a castle; a stronghold; a place of defense or security.
Syn. -- Fortress, Fortification, Castle, Citadel. 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.
1 other Webster entry under this spelling
Fortress, v. t.
To furnish with a fortress or with fortresses; to guard; to fortify. Shak.
Shades of meaning
Fernald has no entry of its own for fortress; he lists it under defense, fortification.
Where he writes about it:
Fortification is the general word for any artificial defensive work; a fortress is a fortification of especial size and strength; a fortress is regarded as permanent, and is ordinarily an independent work; a fort or fortification may be temporary; a fortification may be but part of a defensive system; we speak of the fortifications of a city. — from fortification
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