Drawbridge

n. bridge which can be drawn up or let down to permit or hinder free passage

A drawbridge is a type of movable bridge typically associated with the entrance of a butt the term is often used to describe all different types of movable bridges, like bascule bridges and lift bridges.Drawbridges may be used to withdraw passage across (rather than allow passage beneath), as is found in military fortifications, and this is the most common use of the term. Simple drawbridges may be found in short canal crossings, where the lifting mechanism is a pair of overhead beams with counterweights that are not a part of the bridge's load bearing structure.
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Noun
1. a bridge that can be raised to block passage or to allow boats or ships to pass beneath it
(synonym) lift bridge
(hypernym) bridge, span

(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.
  

The moveable section of a wooden bridge or causeway, often over a moat , operated by counterweights to keep an enemy force from the easy approach to the gatehouse .