A Roman fort that survived the fall of empires

Society · Published 20 October 2026 Swipe more like this →

In short: Portchester Castle is not just a medieval fortress; it is a structural palimpsest. Built within the walls of a 3rd-century Roman fort, it has served as a royal hunting lodge, a site of high-treason arrests, and a prison for thousands. This site represents the most intact Roman military installation situated above the Alps.

The site's strategic value dates back to at least the 3rd century. It is believed that between 285 and 290, the Roman emperor Diocletian instructed Marcus Aurelius Carausius to build a fort here to combat pirate raids. This installation served as a base for the Classis Britannica, the Roman fleet defending Britain. While the Roman army retreated in the early 5th century, the site remained in use on a smaller scale, eventually becoming a Saxon 'burh' in 904 to defend against Vikings.

The medieval castle emerged in the late 11th century. William Maudit, a powerful associate of William the Conqueror, likely established the inner ward, using the original Roman stone walls as an outer defense. By the 1130s, the site was being rebuilt in stone, and an Augustinian priory was established there in 1128. In 1154, the monarchy took control, transforming the site into a royal fortress. It became a favored hunting lodge for King John and a frequent departure point for military campaigns to France.

The castle's history is marked by dramatic shifts in power and purpose. It was captured by the French in 1216 during the reign of King John, only to be recaptured in 1217. In 1415, it was the site where Henry V uncovered the Southampton Plot to overthrow him. By the 17th century, its role shifted toward incarceration. From 1665 onward, it primarily functioned as a prison, holding 500 prisoners during the Second Anglo-Dutch War and eventually housing over 7,000 French prisoners of the Napoleonic Wars in the 19th century.

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