Plymouth’s eastern Hoe already contained coastal defenses before the Royal Citadel was built. Tudor blockhouses guarded approaches to Sutton Harbour, and in the 1590s Sir Francis Drake promoted a larger fort financed partly through a levy on exported pilchards. That work protected a naval and commercial port increasingly important to English operations in the Atlantic. After the Restoration, renewed war with the Dutch exposed the need for stronger defenses capable of commanding Plymouth Sound, the Cattewater, and the harbor while housing a permanent royal garrison.

Charles II’s engineer Sir Bernard de Gomme designed the new citadel, incorporating parts of the Elizabethan fort into an irregular bastioned enclosure. Work began in 1665, and the earl of Bath laid a foundation stone in 1666. Local limestone formed the ramparts, while Portland stone emphasized the monumental Baroque gateway. Six bastions, artillery positions, magazines, barracks, and a dry ditch created a fortress adapted to gunpowder warfare. The medieval chapel of Saint Katherine was rebuilt within the walls as the garrison church.

The citadel’s position carried a political message as well as a military one. Plymouth had supported Parliament during the Civil Wars, and guns on the landward side could dominate the town as well as repel an external attack. This has encouraged the view that Charles II intended the fortress partly to discourage rebellion. Documentary evidence more securely demonstrates its naval purpose, so the balance between defense and domestic control should not be reduced to a single motive. In either case, the royal gateway and commanding walls visibly reasserted restored monarchical authority.

Later generations repeatedly adapted the fortress. Eighteenth-century improvements increased its artillery strength, and by the Victorian period it served the Royal Garrison Artillery as a barracks and training center even after newer forts formed an outer defensive ring around Plymouth. During the Second World War the citadel housed coast-artillery training and command functions while the city endured heavy bombing. Changes to weapons, accommodation, communications, and parade grounds accumulated inside the seventeenth-century perimeter without erasing its essential bastioned plan.

The Royal Citadel remains a working military installation and is associated with 29 Commando Regiment Royal Artillery. Continued use has preserved the broad unity of the fort but necessarily limits ordinary public access, which is generally provided through arranged tours. The ramparts, gateway, chapel, earlier defensive fragments, and later service buildings record more than three centuries of military occupation. Unlike a ruin abandoned after its strategic purpose ended, the citadel illustrates how an early modern fortress can remain active while conservation authorities manage architecture designed for technologies and political conditions long since transformed.