Dover has played a prominent part in English history due to its strategic location at the shortest sea crossing to mainland Europe. Dover Castle is built on the remains of an Iron Age hillfort and contains within its walls a Roman lighthouse and an Anglo-Saxon church.

The first castle was built by Duke William of Normandy in November 1066, shortly after landing at Pevensey, and before the Battle of Hastings. Not much is known about this castle but archaeological evidence suggests it was centred around the Roman lighthouse and Saxon church.

Between 1179 and 1188, Henry II had Dover Castle completely rebuilt. Under the supervision of Maurice the 'Ingeniator' the massive keep and walls and towers of the inner bailey were built and work was started on the walls of the outer bailey. The castle was the first in western Europe to be built to a concentric design (a series of defences, radiating in a succession of larger and larger circles from the central inner ward). The defences were completed by King John after Henry's death.

The castle was put to its first major test during the war between King John and his barons. In May 1216 Prince Louis landed a French Army at Thanet in support of the rebel barons. By the autumn of 1216, the only castles in southern England still in the king's hands were at Windsor and Dover.

Prince Louis set up a camp to the north of Dover Castle from where he could direct the great stone throwing engines that bombarded the outer walls while miners slowly tunnelled under the northern barbican. The undermining of the barbican forced the defenders, led by Hubert de Burgh, to withdraw behind the north gate. When the French miners brought down the eastern of the two gate towers, the French poured into the castle. However de Burgh and his men were ready for them and fought back, forcing the French to retreat through the breach. Faced by a determined castle garrison, Prince Louis called a truce. In October King John died at Newark Castle and his son, Henry III, was proclaimed king. In may of 1217, Prince Louis broke the local truce and once more lay siege to Dover Castle, but three days later French forces were defeated at the Battle of Lincoln, bringing the war to an end.

The siege of 1216-1217 had exposed the vulnerability of the castle's northern defences. With Henry III's backing the castle defences were greatly improved. The northern gateway was blocked solid. In the moat beyond engineers constructed St John's Tower which overlooked a new spur to the north, designed to give the garrison a better command of the high ground. The north gateway was replaced by Constable's Gateway on the western side of the castle. A secondary entrance, Fitzwilliam's Gateway, was built on the eastern side of the castle. The outer curtain wall was completed from Peverell's Tower to the cliff edge, and a massive earth bank was constructed round the church and Roman lighthouse, this was topped by a stone wall in the 1250's. In 1240 a new hall, later to be known as Arthur's Hall, and a set of chambers for the king were built on the south eastern side of the inner ward. By the completion of these works Dover Castle had reached the peak of its Medieval power.

At the outbreak of the Civil War in 1642, Dover town sided with Parliament while the castle garrison supported the king. That August a small party of townsfolk scaled the cliffs, surprised the garrison and captured the castle, which fell with hardly a shot.

In 1745 new barracks were built within the inner bailey to accommodate extra troops. In the 1750's the military engineer J P Desmaretz added further accommodation, part of which was situated in the keep. In 1755 he strengthened the northern defences of the castle, remodelling the outer curtain from Avranches Tower to the Norfolk Towers to carry heavy artillery, modernising the medieval spur to accommodate infantry, and building two gun batteries - Bell Battery and Four Gun Battery.

At the end of the eighteenth century, during the wars with Revolutionary and Napoleonic France, Colonel William Twiss continued the modernisation of the castle. Twiss completed the remodelling of the outer defences adding the huge Horseshoe, Hudson's, East Arrow and East Demi-Bastions to provide extra gun positions on the eastern side, and constructing Constable's Bastion for additional protection on the west. Twiss further strengthened the Spur at the northern end of the castle, adding a redan or raised gun platform. By taking the roof of the keep and replacing it with massive brick vaults he was able to mount heavy artillery on the top. To help troop movements between castle and town defences, Twiss constructed Canon's Gateway. He filled every available space within the castle with barracks and storerooms, and even constructed underground cliff barracks.

The triumphal conclusion of the Napoleonic Wars saw a rapid reduction in Dover's defences; only a small garrison remained at the castle. Later improvements to the fortifications were added, but the castle developed more as a garrison headquarters while the new Fort Burgoyne, built on the high ground to the north east of the castle, took over the other military functions.

The 1850s saw an extensive programme of barrack building, including Salvin's Officers' New Barracks which still dominate the southern part of the castle. The last major rearming was undertaken in the 1870s when a series of gun batteries was built along the cliff edge to protect the harbour below.

In the twentieth century the castle played an important role in both world wars. The castle was armed with anti-aircraft guns and searchlights, supplemented during the Second World War with Radar.

In May 1940, the evacuation of 338,000 allied soldiers from Dunkirk was directed from a command centre in the converted Georgian underground barracks, at Dover Castle. New tunnels were built to house an underground hospital and the combined headquarters for the three services.

After the war the army remained in the castle until 1958; five years later the whole of Dover Castle was handed over to the Ministry of Works for preservation.