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]

The Collaborative International Dictionary of English v.0.48:

Fortress \For"tress\, v. t.
   To furnish with a fortress or with fortresses; to guard; to
   fortify. --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

fortress
    n 1: a fortified defensive structure [syn: fortress, fort]

Moby Thesaurus II by Grady Ward, 1.0:

32 Moby Thesaurus words for "fortress":
   acropolis, bastion, beachhead, blockhouse, bridgehead, bunker,
   castle, citadel, donjon, fasthold, fastness, fort, garrison,
   garrison house, hold, keep, martello, martello tower, mote, motte,
   peel, peel tower, pillbox, post, rath, redoubt, safehold,
   strong point, stronghold, tower, tower of strength, ward