The Collaborative International Dictionary of English v.0.48:

Drawbridge \Draw"bridge`\, n.
   A bridge of which either the whole or a part is made to be
   raised up, let down, or drawn or turned aside, to admit or
   hinder communication at pleasure, as before the gate of a
   town or castle, or over a navigable river or canal.
   [1913 Webster]

   Note: The movable portion, or draw, is called, specifically,
         a bascule, balance, or lifting bridge, a turning,
         swivel, or swing bridge, or a rolling bridge, according
         as it turns on a hinge vertically, or on a pivot
         horizontally, or is pushed on rollers.
         [1913 Webster]

WordNet (r) 3.0 (2006):

drawbridge
    n 1: a bridge that can be raised to block passage or to allow
         boats or ships to pass beneath it [syn: drawbridge, lift
         bridge]

Moby Thesaurus II by Grady Ward, 1.0:

81 Moby Thesaurus words for "drawbridge":
   Bifrost, abatis, advanced work, balistraria, bank, banquette,
   barbed-wire entanglement, barbican, barricade, barrier, bartizan,
   bascule bridge, bastion, bateau bridge, battlement, breastwork,
   bridge, bulwark, cantilever bridge, casemate, catwalk,
   cheval-de-frise, circumvallation, contravallation, counterscarp,
   curtain, demibastion, dike, earthwork, enclosure, entanglement,
   escarp, escarpment, fence, fieldwork, floating bridge, flyover,
   footbridge, fortalice, fortification, gangboard, gangplank,
   gangway, glacis, lift bridge, loophole, lunette, machicolation,
   mantelet, merlon, mound, outwork, overbridge, overcrossing,
   overpass, palisade, parados, parapet, pontoon bridge, portcullis,
   postern gate, rampart, ravelin, redan, redoubt, rope bridge,
   sally port, scarp, sconce, span, stepping-stone, stepstone,
   stockade, suspension bridge, swing bridge, tenaille, toll bridge,
   vallation, vallum, viaduct, work