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.
          [1913 Webster]