The Collaborative International Dictionary of English v.0.48:
Fortress \For"tress\, n.; pl. Fortresses. [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.
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.