Portchester Castle began as a vast Roman fort at the head of Portsmouth Harbour. Built in the later third century, probably after about 268, it is commonly identified with Portus Adurni, one of the commands listed among the Saxon Shore defenses. Its nearly complete circuit of walls and projecting towers protected a natural harbor and enclosed homes, workshops, pens, and military facilities. The Roman masonry survives to exceptional height, making Portchester the best-preserved Roman fortification of its kind in northern Europe.

Occupation continued after Roman rule ended. Archaeologists have found early Anglo-Saxon huts, a well, fields, houses, and a cemetery within the walls. In 904 the Bishop of Winchester transferred the fort to King Edward the Elder, and it became a burh within the kingdom’s defenses against Viking attack. A large hall and stone tower suggest elite residence in the tenth century. Portchester’s continuous reuse depended on practical advantages: secure walls, fresh water, farmland, and access to one of southern England’s finest anchorages.

After the Norman Conquest, William Mauduit established a castle in the northwestern corner of the Roman enclosure. A stone keep rose in stages during the twelfth century, and an Augustinian priory founded inside the fort in 1128 left its church as today’s parish church. The Crown recovered the castle under Henry II and used it as a residence, treasury, prison, and embarkation point for France. Richard II constructed an impressive palace there in the 1390s, adapting the confined inner bailey for royal ceremony.

English armies repeatedly assembled at Portchester. Henry V was there in 1415 as preparations advanced for the campaign that culminated at Agincourt, and the Southampton Plot against him was exposed nearby. Portsmouth’s growth eventually reduced the castle’s strategic value. From 1665 the government instead used it to confine prisoners of war. During the French Revolutionary and Napoleonic wars it held thousands, including Black and mixed-race prisoners brought from the Caribbean in 1796 and French captives who created a theater in the keep.

The army returned the castle to its owners in 1819, after which the ruins attracted visitors and antiquarians. It entered state guardianship in 1926. Major excavations directed by Barry Cunliffe between 1961 and 1979 clarified Roman, Saxon, and medieval occupation, while later research identified the prisoners’ buildings and experiences. Portchester’s power lies in this accumulation: Roman walls became a Saxon settlement, Norman fortress, royal palace, military port, and prison. Conservation now preserves those overlapping stories rather than privileging only the monumental keep.