Pevensey Castle began as the late Roman fortress of Anderitum, built around 290 CE on a peninsula that then projected into tidal marshes. Its irregular oval walls enclosed a broad area and formed part of the coastal defenses later known as the Saxon Shore forts. Thick masonry, projecting towers, and commanding views protected a harbor and possible naval base. Archaeology suggests continuing occupation into the early fifth century, with soldiers, craftspeople, families, livestock, workshops, and rubbish deposits sharing the defended enclosure.

People continued to use the Roman walls after imperial government ended. Evidence for early Anglo-Saxon huts, wells, farming, and later timber buildings shows that the fort did not simply become empty. By the tenth century Pevensey was part of a fortified network protecting the English kingdom. Its best-known turning point came in September 1066, when William, Duke of Normandy, landed nearby. His army erected temporary defenses within the Roman circuit before moving toward Hastings and victory over Harold II.

Norman lords converted the eastern side of the Roman fort into a compact medieval castle. Earth-and-timber works gave way to stone walls, towers, a gatehouse, and a substantial keep. Because Pevensey controlled a landing place and route inland, it faced repeated attack during conflicts between kings and nobles. Sieges in the twelfth, thirteenth, and fifteenth centuries led to repairs and adaptations. The Roman enclosure remained the outer bailey, demonstrating how medieval builders exploited defenses already hundreds of years old.

The castle also served as a secure prison for high-status captives, including King James I of Scotland before his accession. Silting changed the coastline and reduced Pevensey’s maritime importance, while new weapons made the fortress less useful. By the Tudor period many buildings were decaying, though the walls remained conspicuous. Antiquarian interest and archaeological investigation later revealed the site’s unusually long sequence. Excavated coins, pottery, animal bones, building foundations, and military objects helped reconstruct lives largely absent from written chronicles.

Pevensey returned to military service during the Second World War, when invasion again seemed possible. Concrete pillboxes, gun positions, and shelters were built into or disguised against the ancient walls, and British, Canadian, and American troops occupied the site. Those additions are now preserved as part of its history. Managed by English Heritage, Pevensey presents Roman fort, Norman castle, medieval prison, picturesque ruin, and modern defensive post together. Few sites show so clearly how one strategic enclosure could be reinterpreted across seventeen centuries.