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The drawbridge is a simple Resistance contraption used in City 17 during the Uprising, allowing Rebels to move between buildings above ground level and also prevents Combine units from following them. It is first seen in the Half-Life 2 chapter Anticitizen One and is seen again in Episode One, during the chapter Urban Flight.
In the chapter Anticitizen One, the drawbridge spans between two apartments. Here Gordon discovers that a major Resistance base is under attack. The bridge can be seen when Resistance members are retreating to an apartment, when it is lifted via a switch in order to prevent Overwatch forces from crossing.
It is necessary for Gordon to use several drawbridges in order to reach his rendezvous with Alyx with his squad of Rebels behind him.
Half-Life 2: Episode One
In the chapter Urban Flight, the drawbridge is seen again when RPG-armed Rebels are attacking Overwatch ground forces. Gordon later advances to a base that is trying to reach the Technical Trainstation. There Barney gives him another crowbar that was blocking the last drawbridge in the open position.