What does "drawbridge" mean?

Drawbridge: 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. 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.