How Do You Make a Bridge That Stops Working? Raising the Way In

A fortification with a ditch or moat around it needs a way in, and that way in is the weakest point by definition, since it is the one place where the barrier is deliberately absent. Every device at a gatehouse addresses that. A bridge that can be removed converts the entrance back into part of the ditch, which forces an attacker to fill the ditch, to bring their own bridge or to attack elsewhere, each of which is slow and exposed. It also controls who enters in ordinary times, which mattered more day to day than defence against a siege did, since most castles spent most of their existence not being attacked.

Several mechanisms were used and each has a distinct signature in surviving stonework:

• •Chains and a windlass, hauling the far end upward against the gate
• •Counterweights on beams pivoting above the gate, which reduce the effort enormously
• •A pivoting deck, balanced so that one end rises as the other drops into a pit
• •Slots in the stonework above the gate, which received the lifting beams and are the commonest surviving evidence
• •Recesses in the face of the gatehouse into which the raised bridge fitted flush
• •Turning bridges that rotated horizontally, used where a lifting arrangement was impractical

The bridge is one element of a layered arrangement and the others are frequently confused with it. A portcullis is a heavy grille dropped vertically in grooves, which can be released instantly and which is quite separate from any bridge. Murder holes in the ceiling of the passage allowed defenders above to attack anybody trapped between the outer and inner barriers, and the arrangement is deliberately designed to produce that trap. Doors of heavy timber reinforced with iron close behind. A barbican extends the approach outward, forcing attackers along a confined route under fire. Bends in the passage prevent a straight charge and prevent a ram being used. The sequence means an attacker must defeat several distinct obstacles in succession under attack throughout.

Lifting bridges did not disappear with castles and the modern versions solve a different problem. A bridge crossing a navigable waterway must either be high enough for vessels to pass beneath, which is expensive and steep, or must move. Bascule bridges pivot upward on counterweighted leaves and are the direct descendants of the castle arrangement, with the counterweight below the roadway doing exactly what a castle counterweight did. Swing bridges rotate horizontally on a central pier. Vertical lift bridges raise a whole span between towers. Transporter bridges carry a platform across on cables, avoiding the need to move the deck at all, and a handful survive. Each is a compromise between construction cost, the height of vessels and how often the crossing must open.

Working examples are extremely rare and the reasons are straightforward. The moving parts were timber and iron, which rot and corrode, and nothing maintained them once the castle stopped being defended. Moats were drained and ditches filled, which removed the purpose. Gatehouses were converted into residences and the openings blocked or widened. Where a bridge was replaced, it was replaced with a fixed one. What survives is therefore the stonework, with the slots, recesses, pivot points and counterweight pits still legible, which is how the mechanism at a particular site is reconstructed. Several working reconstructions have been built at castles open to visitors, based on that evidence and on manuscript illustrations.

The entrance is the deliberate gap in a barrier, and a bridge that lifts converts it back into part of the ditch. Chains and windlasses, counterweighted beams and pivoting decks were all used, and the slots and recesses they needed are what survives in the stonework. A portcullis, murder holes and a bent passage are separate obstacles in the same layered arrangement.