In 1216, a major French invasion put Henry’s castle to the test–and it nearly failed. Prince Louis of France, in alliance with English barons rebelling against King John, invaded south-east England. Like previous invaders, the French landed on easy ground and marched inland, quickly gaining control of the entire district–except Dover Castle. Hubert de Burgh, John’s fierce Justiciar, held Dover Castle for the King with only 140 knights but ample provisions. The French invaders quickly discovered the castle’s weak point: an area of high ground outside the North Gate where they could avoid the downward fire from the defenders. From there the French breached the outer wall by undermining the gate and letting it collapse into the tunnel. But the inner walls held, and de Burgh’s knights pushed the French back through the breech. This blunted Prince Louis’ appetite for storming the great fortress, and the invasion fizzled.

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Under a new king, Henry III, de Burgh set about altering the battered castle to prevent another such attack. He completely blocked the failed North Gate and threw up great outworks beyond it to deny the high ground to any future invaders. He linked the outworks to the castle with a maze of underground passages–the Medieval Tunnels that fascinate visitors today. De Burgh then replaced the North Gate with the splendid Constable’s Gateway, protected by no fewer than six overlapping towers, and still the residence of the Deputy Constable of Dover Castle. Finally, he extended the outer wall and its towers all the way to the cliff’s edge, an enormous distance. When de Burgh’s alterations were done, he had created the medieval castle we see today.

After 1500, gunpowder weapons made Dover Castle increasingly obsolete, and its garrison slowly dwindled. Then, in 1744, French Jacobites began to pose a serious threat of invasion, and Dover Castle came alive. As the garrison swelled, engineers added gun batteries and replaced the medieval buildings that lined the inner bailey with barracks. These elegant barracks, the oldest in England, still ring the austere medieval keep. But these changes, the first military additions to Dover in five centuries, were just a prelude to those made during the Napoleonic Wars. As Napoleon massed an invasion force of 100,000 troops within sight of Dover, the castle’s military engineers scrambled frantically to strengthen a castle built to withstand arrows and rocks.

These modernization efforts encrust every corner of the castle. The engineers strengthened the outer wall with earthen ramparts and mounted guns on them. They replaced the keep’s medieval roof with a vaulted brick one and placed additional guns there, too. Medieval towers that blocked the artillery’s fields of fire were partially dismantled. Guns were placed all over the Western Heights opposite the castle, mounted in an elaborate network of redoubts, batteries, and tunnels. Further gun and infantry positions sprang up all over the slopes below the castle, ringing it with elaborate outworks reached via brick tunnels. Many of these outworks consist of long galleries set in the hillside, with gun loops from which riflemen could sweep attacking infantry off the adjoining slopes. Finally, the defenders mounted guns on the medieval outwork by converting it into an elaborate brick redan, or gun platform, with intricate underground tunnels. The redan tunnels, linked to the original medieval tunnels, are open to the public, and other Napoleonic outworks and tunnels may be opened in the future.

The Napoleonic engineers put up barracks and support structures every place they could find, but still they ran out of room. In 1797, they went underground, building an elaborate system of underground barracks, seven parallel brick vaults of great length and height linked by a tunnel to their rear. At the rear the vaults are 50 feet underground, but in front, along the cliff face, a balcony offers sweeping views over Dover Harbour. Called ‘the Casements’, they were first occupied in 1803 and held 2,000 soldiers.

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