drawbridge.
1. Bridge over a moat, ditch, river, etc., hinged at one end, like a flap, fixed to chains at the other end, capable of being raised and lowered to prevent or permit passage over it, especially in fortifications.

2. The same, but over a waterway, raised to permit the passage of ships beneath it. In such cases the flap is often matched with an identical flap on the other side, and each rotates about an axis, counterweighted at the other side of the axis. This type is a bascule bridge.

draw·bridge / ˈdrôˌbrij/ • n. hist. a bridge, esp. one over a castle's moat, that is hinged at one end so that it may be raised to prevent people's crossing or to allow vessels to pass under it.