Dover's castle hill commands the narrowest part of the English Channel and contains evidence from many defensive eras. An Iron Age enclosure may have occupied the height. The Romans built a lighthouse, or pharos, in the second century CE to guide ships toward their port; one substantial tower survives beside the later church of St Mary in Castro. Anglo-Saxon use is less completely understood, but the church probably dates principally from around the year 1000. After the Norman Conquest, William I strengthened Dover, although little of his first fortification remains visibly distinct.

The dominant medieval castle was created by Henry II between 1179 and 1189. Its enormous great tower combined royal apartments, ceremonial display, administration, storage, and defense within a concentric system of walls and gates. Henry's expenditure may have related both to Channel security and to receiving elite travelers bound for Thomas Becket's shrine at Canterbury after the archbishop's murder. Under King John and Henry III, outer defenses expanded. In 1216 Hubert de Burgh's garrison resisted forces supporting Prince Louis of France, and siege damage prompted further strengthening.

Gunpowder, changing frontiers, and recurrent war kept Dover active after medieval military architecture became obsolete elsewhere. Tudor and later engineers adapted defenses, while the Napoleonic threat produced major earthworks, batteries, barracks, and tunnels cut into the chalk. Those tunnels housed soldiers on a scale that amounted to an underground barracks. Nineteenth-century works integrated the old castle into a wider fortified landscape protecting harbor and strait. The result is not a purely Norman fortress but a layered military complex in which successive technologies reused the same commanding ground.

During the Second World War, the tunnels became a naval command center and hospital. Vice-Admiral Bertram Ramsay directed Operation Dynamo, the 1940 evacuation from Dunkirk, from rooms beneath the castle. Later planning, communications, and coastal defense again connected Dover to cross-Channel operations. In the Cold War, parts of the underground complex were prepared as a regional government center in case of nuclear attack. Modern presentation recreates some wartime environments through objects, sound, and interpretation; these displays should be distinguished from original tunnel fabric and surviving service installations.

English Heritage now manages Dover Castle with ticketed access to the great tower, walls, grounds, church, Roman lighthouse, and selected tunnels. Extensive medieval masonry survives, but roofs, interiors, and royal chambers have undergone conservation and interpretive refurnishing. The pharos contains original Roman work altered by medieval use, while underground spaces reflect Napoleonic, wartime, and Cold War adaptation. Dover is therefore neither an intact 1066 castle nor a single-period reconstruction. Its exceptional value comes from nearly two millennia of strategic reuse at one location, with authentic fabric from several eras connected by the continuing importance of the Channel crossing.