How Did Medieval Castles Work? Walls, Sieges and the End of the Fortress

The first castles were quick. A motte-and-bailey was a mound of earth, the motte, thrown up in weeks by forced labour, with a wooden tower on top and a fenced yard, the bailey, at its foot; William the Conqueror's men built dozens across England to hold a hostile country, and the Bayeux Tapestry shows one going up. Wood rots and burns, and from the late eleventh century the towers were rebuilt in stone as keeps, square blocks with walls three or four metres thick and the entrance on the first floor, as at the Tower of London and Rochester. A keep was a last refuge more than a fighting platform, and its corners were its weakness: at Rochester in 1215 King John's miners brought down one of them by burning forty pigs' worth of fat under the foundations, and the corner was rebuilt round.

The crusaders came home from the east with new ideas, and the castles of the thirteenth century put the strength in the walls rather than the keep. Edward I's castles in Wales, built in the 1280s by a Savoyard master mason, are the type: a ring of high curtain wall studded with round towers that let archers shoot along the wall's face, inside a lower outer ring, so that attackers who took the outer wall stood in a killing ground below the inner one. The gate, the weakest point, became the strongest, a gatehouse with several sets of doors and portcullises, murder holes in the ceiling and a drawbridge in front. The features and what they were for:

• •Round towers: no corners to mine, and no blind spots for defenders
• •Arrow slits: narrow on the outside, splayed on the inside, so an archer could aim widely while presenting a target a few centimetres across
• •Machicolations: overhanging galleries at the top of the wall with holes in the floor, for dropping stones on anyone at its base
• •The moat: water or a dry ditch, which stopped siege towers and rams reaching the wall and made mining under it impossible
• •The barbican: an outwork in front of the gate that forced attackers to approach along a channel under fire

A castle was rarely stormed. The usual siege was a blockade, in which the attacker surrounded the place and waited for the food, water or will to run out, while trying to shorten the wait. Trebuchets, counterweight catapults that could throw a hundred-kilogram stone a couple of hundred metres, battered the walls and lobbed dead animals over them to spread disease; siege towers rolled up to the wall on wheels so that troops could cross at the top; battering rams under roofs of wet hides attacked gates; and miners dug tunnels under a tower, propped them with timber, and burned the props so that the wall above collapsed, which the defenders answered by counter-mining and fighting underground by candlelight. A well-supplied castle could hold for months, and a besieging army, sitting in its own filth, often died faster than the garrison; many sieges ended by negotiation, with the garrison marching out under terms if no relief had come by an agreed date.

In peacetime, which was most of the time, a castle was a manor house with thick walls. The great hall was the centre, where the household ate, the lord held court and the servants slept on the floor; the lord's family had chambers in the towers, with fireplaces, latrines emptying down the wall and, by the fourteenth century, glazed windows on the sheltered side. There were kitchens, a chapel, stables, a well, which was the one thing a castle could not do without, storerooms for a year's grain, and a prison in whichever tower was dampest. A castle's garrison in peace might be a dozen men, and a great magnate with twenty castles kept each with a skeleton staff and moved between them with his household, eating his way through the produce of each estate in turn.

Cannon arrived in the fourteenth century and by the fifteenth could bring down a curtain wall in days; the French artillery train of the 1450s took the English castles of Normandy at a rate of one a week. High, thin walls were exactly the wrong shape for resisting a ball, and the response, worked out in Italy around 1500, was the low, thick, angular bastion fort with earth-backed ramparts that absorbed shot, which is a fortress but not a home. The lords kept their castles as houses and status symbols, added windows and gardens, and built new houses without walls at all. The English Civil War was the last time the old castles were fought over, and Parliament slighted most of them afterwards, blowing out a wall so they could not be held again, which is why so many stand as picturesque ruins rather than as buildings.

A stone castle was one of the largest construction projects of its age, and a surprising amount is known about how they were built because royal clerks recorded the spending. The English crown's accounts for Edward I's Welsh castles list payments to masons, carpenters, quarrymen, ditch diggers and carters, summoned from counties all over England and numbering in the thousands on a single site at the peak of a building season. Work usually slowed or stopped in winter, when frost could ruin fresh lime mortar, and the tops of unfinished walls were covered over until spring.

The walls themselves still carry evidence of the process. Rows of small square holes in the stonework are putlog holes, which held the timber beams of scaffolding and were left open or loosely filled when the scaffold came down. Many castles were also far brighter than their bare grey ruins suggest: walls were often rendered and whitewashed, and the White Tower at the Tower of London takes its name from being whitewashed in the thirteenth century. A castle was meant to be seen across the landscape, and a gleaming wall announced its owner's power as clearly as a thick one did.

Several favourite images of castle warfare are exaggerated. Boiling oil poured from the battlements appears in countless films, but oil was costly and medieval records of it being used are rare; defenders far more often dropped stones, which cost nothing and did the job. The murder holes in the ceiling of a gate passage could certainly rain missiles on attackers trapped between two doors, but many historians think they were just as useful for pouring water down to put out fires set against the timber gates, which was one of the commonest ways of attacking an entrance.

Even the word dungeon has drifted. It comes from the French donjon, the name for the great tower or keep, which is linked to the Latin word for lordship, because the keep was the symbol of the lord's power. Prisoners were often held in its lowest storey, and over time the name of the whole tower attached itself to the cells beneath it. The deep pits shown to visitors as oubliettes, where prisoners were supposedly dropped and forgotten, often turn out to be something more ordinary, such as storerooms, cisterns or latrine shafts, given a grim story by Victorian guides who enjoyed a good horror.

Bodiam Castle in East Sussex looks like a perfect medieval fortress, a square of towers rising from a broad moat. It was built in the 1380s by Sir Edward Dalyngrigge, a veteran of the Hundred Years War, with royal permission to fortify it against possible French raids. For a long time it was described as a serious defensive work.

Later historians pointed out weaknesses, such as relatively thin walls, a moat that could be drained and windows that would have been vulnerable, and argued that Bodiam was as much a statement of wealth and status, designed to impress visitors approaching across a landscaped setting of water. The debate continues, and it shows that castles often served as symbols of power as well as places to defend.

A medieval castle was a lord's fortified residence and the base from which he held the surrounding land, evolving from Norman earth mounds with timber towers to stone keeps and then to concentric rings of towered curtain wall with fortified gatehouses, designed to make attack slow and expensive. Sieges were mostly blockades helped by trebuchets, towers, rams and mining, and the castle's age ended when cannon could breach its walls, after which fortresses became low and thick and lords moved into houses.