The 10 strongest castles in England

The 'Key of England,' Dover is the great fortress of the Channel coast: Henry II's mighty square keep, among the largest in Britain, set inside two concentric rings of curtain wall and mural towers on the white cliffs above the shortest crossing to France. Scale and position both work for it, the ground falls away to the sea, and an attacker must fight through ring after ring.

No besieging army ever took it. In 1216–17 Prince Louis's French miners tunnelled under the northern defences and brought down a gate tower, but Hubert de Burgh's garrison plugged the breach and threw them back, and the siege failed. It changed hands only once, in August 1642, and then by stealth, when townsmen scaled the cliffs and surprised the garrison for Parliament.

Kenilworth's strength was water. The Norman great keep and curtain sat behind the Great Mere, a broad artificial lake held back by a dammed causeway, the most extensive water defences of any English castle, turning the place into an island no ram or siege tower could reach.

That system produced the longest siege in English history. In 1266 Henry III and the Lord Edward invested the Montfortian garrison for some 172 days, from June to 13 December, hurling stones with trebuchets and even floating barges of men across the mere, all beaten off by the water and the walls. The castle was never stormed; it gave in only when starvation and disease forced a negotiated surrender.

The Tower of London is a first-rank concentric fortress: the Norman White Tower at its core, wrapped by Richard I, Henry III and Edward I in two rings of curtain wall, thirteen mural towers and a broad wet moat fed from the Thames. On paper few English castles are better defended.

Its actual war record is thinner than its walls. The stronghold was breached in June 1381 when Peasants' Revolt rebels walked through gates left open during Richard II's absence and dragged Archbishop Sudbury to his death, the only time it was taken by force. In 1460 Yorkist besiegers battered the curtain with bombards and starved the Lancastrian garrison into surrender on 19 July. Its strength lay in dominating London; it fell more to tumult and blockade than to any failure of its stone.

Warwick pairs a strong natural line, a sandstone cliff dropping to the River Avon on its river face, with some of the finest military architecture in England: Thomas Beauchamp's 14th-century gatehouse and barbican, machicolated and portcullised, flanked by the great Caesar's and Guy's Towers rising up to 45 metres.

Though a show-piece stately home today, those defences were built to fight, and they held in the Civil War. A Royalist force laid siege on 7 August 1642, but Sir Edward Peyto's small garrison kept them out until the Earl of Essex relieved the castle on 23 August. It was not taken.

Bamburgh crowns a great whinstone crag rising sheer above the Northumberland coast, one of the strongest natural fortress sites in England, the sea on one flank, a long climb on the others, walled along the rock with a Norman keep at its heart. For centuries it was reckoned all but impregnable.

That reputation ended in 1464. At the close of the Wars of the Roses the Earl of Warwick's guns pounded the walls and brought down a tower, and the Lancastrian garrison surrendered, the first castle in England ever taken by gunpowder artillery. Its position, more than its masonry, had always been the real defence, and once cannon arrived the position was no longer enough.

Rochester guards the Medway crossing on the old Watling Street road to London behind one of the tallest Norman keeps in England, some 34 metres high, its walls up to 3.6 metres thick and split by an internal cross-wall so each half could fight on alone. It was built to be a hard nut, and in 1215 it proved it: rebel barons held it against King John for close to two months. John undermined the outer wall, then tunnelled beneath the keep and fired the mine props with the fat of forty pigs, bringing the south-east corner crashing down. Even then the garrison fought on behind the cross-wall until hunger forced surrender. Strong as it was, Rochester fell, and its bailey was stormed again in 1264.

Corfe stands on a steep, isolated knoll filling a natural gap in the Purbeck ridge, a site so commanding that the Normans made it a royal stronghold and King John kept treasure and prisoners here behind its stone keep and curtain. It earned its name in the Civil War. In 1643 Lady Mary Bankes and a handful of men beat off a Parliamentarian force several hundred strong. It held out again through a long siege in 1645 and was never stormed: it fell only by treachery, when a defending officer smuggled enemy troops inside disguised as reinforcements and opened the gates. Parliament then slighted it with gunpowder, which is why it stands in ruins today.

Carlisle is England's great border castle, a compact red-sandstone stronghold with a squat Norman keep that stood on the front line of three centuries of Anglo-Scottish war, the most besieged place in Britain and Ireland, attacked by the Scots seven times between 1173 and 1461. Its defences were sturdy rather than elaborate; its real strength was endurance. It saw off Robert the Bruce, who invested it from 22 July to 1 August 1315 and withdrew empty-handed the year after Bannockburn. It was not impregnable, though: in November 1745 it surrendered to Bonnie Prince Charlie's Jacobites after a brief siege, before the Duke of Cumberland's guns quickly took it back.

Framlingham was a technically advanced fortress: instead of a central keep it relied on a tall, continuous curtain wall strung between thirteen mural towers, with a fighting wall-walk running unbroken around the whole circuit, an early English experiment in defending the perimeter rather than a strongpoint. As architecture it impresses; as a war record it disappoints. When King John marched on Roger Bigod's seat in March 1216, the garrison surrendered after just two days, having taken warning from the battering John had just given Rochester. The walls were never seriously tested. Its place here rests on the sophistication of its design, not on any siege it withstood.

Windsor began as a strong motte-and-bailey guarding the Thames west of London, its Round Tower raised on a great earthwork and its wards ringed by curtain walls, and it once fought like a fortress: in 1216 the constable Engelard de Cigogné held it for King John through a roughly two-month siege by rebel barons and the French, who battered the lower ward but never took the place. Since then, though, Windsor has become first and foremost a royal palace, its walls and towers rebuilt for majesty rather than defence, most drastically in Wyatville's 19th-century remodelling. For all its fame, it is the least true fortress of these ten, which is why a ranking by fighting strength places it last.