Drawbridge

A drawbridge is a type of moveable bridge used for defense.

These bridges are most famous for being at the entrance to castles or towers. Many of these strong structures were surrounded by a moat. A moat is a deep ditch that often contains water. The drawbridge provides a way to cross this ditch to reach the gate. When the bridge is lowered, it allows people to enter the building. When it is raised, it denies entry to enemy forces. This makes the bridge a vital part of a defensive structure.

Most historical drawbridges worked using a hinge or a pivot. One common design featured a wooden deck hinged at the inner edge of a gatehouse. This edge sat at the gatehouse threshold. To move the bridge, people used ropes or chains. These were attached to a windlass, which is a machine used for winding. The windlass was located in a chamber inside the gatehouse above the passage. Because lifting a heavy bridge is difficult, many designs used a bascule arrangement. A bascule arrangement uses weights to help balance the load.

There were several ways to manage the weight of the bridge. Some bridges extended into the gate-passage beyond the pivot point. This part might swing over a pit to create an extra obstacle. Other designs used counterweighted beams that dropped into slots in the floor. Sometimes, the raising chains were attached directly to counterweights. In certain locations, like Alnwick, a portcullis provided the necessary weight. The portcullis is a heavy, sliding gate that helps secure the entrance. These mechanisms ensured the bridge could be raised quickly during an attack.

By the 14th century, a more advanced bascule system became common. This system used lifting arms known as gaffs. These gaffs sat above and parallel to the bridge deck. Chains linked the ends of the gaffs to the lifting part of the bridge. When the bridge was raised, the gaffs fit into slots in the gatehouse wall. These specific slots are called rainures. You can still see these rainures at Herstmonceux Castle. The gaffs might also hold counterweights to assist the lifting process. They could even form the side-timbers of a stout gate.

Another type of moveable bridge is the turning bridge. A turning bridge pivots on central points called trunnions. These bridges might include the chains found on a standard drawbridge. The inner end of a turning bridge often carried counterweights. These weights allowed the bridge to sink into a pit within the gate-passage. When the bridge was in a horizontal position, stout pegs supported it. These pegs were inserted through the side walls. However, this arrangement was often clumsy. Many turning bridges were eventually replaced by more advanced drawbridge designs.

We can still see examples of these historic engineering feats today. In France, working drawbridges exist at several châteaux, such as the Château du Plessis-Bourré. In England, Helmingham Hall has two working drawbridges in regular use. These date back to the early sixteenth century. Other examples include the fort of Ponta da Bandeira in Lagos, Portugal. In the Netherlands, there is a double-beam drawbridge called the Poortbrug in Leeuwarden. These surviving structures show how different cultures solved the problem of defense.

Drawbridges also appear frequently in modern films as part of castle sets. Building a functional drawbridge for a movie presents engineering challenges. A film set might not be able to support a heavy bridge. To solve this, engineers often use steel and concrete for the structure. They then hide these strong materials behind wood and plaster. This allows the bridge to look realistic while remaining safe. This connection between historical defense and modern engineering shows the lasting impact of the drawbridge design.