drawbridge -
A hingedbridge which can be raised to prevent its being crossed, as across a moat, or to allow watercraft to travel beneath it.
Wikipedia
A drawbridge is a type of movable bridge typically associated with the entrance of a castle surrounded by a moat. The term is often used to describe all different types of movable bridges, like bascule bridges and lift bridges.
OmegaWiki Dictionary
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drawbridge A movable bridge which can be lifted in order to block the entrance to a castle.