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]