The Collaborative International Dictionary of English v.0.48:

Bulwark \Bul"wark\, v. t. [imp. & p. p. Bulwarked; p. pr. &
   vb. n. Bulwarking.]
   To fortify with, or as with, a rampart or wall; to secure by
   fortification; to protect.
   [1913 Webster]

         Of some proud city, bulwarked round and armed
         With rising towers.                      --Glover.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Bulwark \Bul"wark\, n. [Akin to D. bolwerk, G. bollwerk, Sw.
   bolwerk, Dan. bolv[aum]rk, bulv[aum]rk, rampart; akin to G.
   bohle plank, and werk work, defense. See Bole stem, and
   Work, n., and cf. Boulevard.]
   1. (Fort.) A rampart; a fortification; a bastion or outwork.
      [1913 Webster]

   2. That which secures against an enemy, or defends from
      attack; any means of defense or protection.
      [1913 Webster]

            The royal navy of England hath ever been its
            greatest defense, . . . the floating bulwark of our
            island.                               --Blackstone.
      [1913 Webster]

   3. pl. (Naut.) The sides of a ship above the upper deck,
      usually a fencelike structure around the deck.
      [1913 Webster]

   Syn: See Rampart.
        [1913 Webster]

WordNet (r) 3.0 (2006):

bulwark
    n 1: an embankment built around a space for defensive purposes;
         "they stormed the ramparts of the city"; "they blew the
         trumpet and the walls came tumbling down" [syn: rampart,
         bulwark, wall]
    2: a fencelike structure around a deck (usually plural)
    3: a protective structure of stone or concrete; extends from
       shore into the water to prevent a beach from washing away
       [syn: breakwater, groin, groyne, mole, bulwark,
       seawall, jetty]
    v 1: defend with a bulwark

Moby Thesaurus II by Grady Ward, 1.0:

176 Moby Thesaurus words for "bulwark":
   abatis, abutment, advanced work, aegis, arc-boutant, arch dam,
   arm guard, backstop, balistraria, bamboo curtain, bank, banquette,
   bar, barbed-wire entanglement, barbican, barrage, barricade,
   barrier, bartizan, bastion, battlement, beam, bear-trap dam,
   beaver dam, boom, breakwater, breastwork, brick wall, buffer,
   bulkhead, bumper, buttress, buttress pier, buttressing, casemate,
   cheval-de-frise, circumvallation, citadel, cofferdam,
   contraceptive, contravallation, copyright, counterscarp, cover,
   crash helmet, curtain, cushion, dam, dashboard, defend, defense,
   demibastion, dike, ditch, dodger, drawbridge, earthwork,
   embankment, enclosure, entanglement, escarp, escarpment, face mask,
   fence, fend, fender, fieldwork, finger guard, flying buttress,
   foot guard, fort, fortalice, fortification, fortress, fuse, gate,
   glacis, goggles, governor, gravity dam, groin, guard, guardrail,
   hand guard, handrail, hanging buttress, hard hat, helmet,
   hydraulic-fill dam, insulation, interlock, iron curtain, jam,
   jetty, jutty, knee guard, knuckle guard, laminated glass,
   leaping weir, levee, life preserver, lifeline, lightning conductor,
   lightning rod, logjam, loophole, lunette, machicolation, mantelet,
   mask, merlon, milldam, moat, mole, mound, mudguard, nose guard,
   outwork, pad, padding, palisade, palladium, parados, parapet,
   patent, pier, pier buttress, pilot, portcullis, postern gate,
   preventive, prophylactic, protect, protective clothing,
   protective umbrella, rampart, ravelin, redan, redoubt,
   retaining wall, roadblock, rock-fill dam, safeguard, safety,
   safety glass, safety plug, safety rail, safety shoes,
   safety switch, safety valve, sally port, scarp, sconce, screen,
   seat belt, seawall, secure, shelter, shield, shin guard, shoulder,
   shutter dam, stockade, stone wall, stronghold, sun helmet,
   tenaille, umbrella, vallation, vallum, wall, weir, wicket dam,
   windscreen, windshield, work