Portchester Castle is a medieval castle and former Roman fort at Portchester to the east of Fareham in the English county of Hampshire. It is located at grid reference SU624045, occupying a commanding position at the head of Portsmouth Harbour. The castle is a Scheduled Ancient Monument, and a Grade I listed building. The castle has been in the ownership of the Southwick Estate since the 17th century but is managed by English Heritage and open to visitors throughout the year. It is the only Roman fort in northern Europe with the majority of the walls and bastions complete and still standing.
The Norman church, St. Mary's, which stands in the south-east corner of the grounds, falls within the Anglican Diocese of Portsmouth.
It is thought likely the fort of Portus Adurni mentioned in the Notitia Dignitatum is Portchester Castle and may have been a base of the Classis Britannica. It was built on the site of the British fortification Caer Peris, possibly during the 3rd century to protect the southern coastline of Roman Britain (litus saxonicum, the Saxon Shore), from Saxon raiders.
There are many different opinions of when exactly the fort was constructed, but coin excavations suggest that the castle was built by Marcus Aurelius Carausius on the instructions of emperor Diocletian between AD 285 and 290.
Around 1090, William Mauduit, the lord of the local manor, built a small Norman castle in the north-west corner of the fort, with a single storey stone keep and wooden palisade on two sides. The old Roman walls became the outer bailey. William Pont de l'Arche acquired the castle in 1130 and founded the priory in the south-east corner of the outer bailey. By 1158, the castle had reverted to the Crown and, about 1180, the palisade was replaced with stone walls surrounded internally by domestic buildings. King John often stayed at Portchester Castle and was there when he heard of the loss of Normandy in 1204. There was major rebuilding work during the 14th century and Edward III assembled his 15,000 strong army there before leaving for France and victory at the Battle of Crecy. Richard II turned the castle into a magnificent palace, and Henry V spent the night at the castle before travelling to Southampton, thence France and the Battle of Agincourt. He undertook a Fleet Review at Portchester before leaving.
After Henry VII founded the Royal Dockyard at Portsmouth, the castle lost its importance. It was last used in the 19th century as a gaol for over 7000 French prisoners of the Napoleonic Wars. Hospital Lane (formerly Seagates Lane), which flanks the western side of the castle, was the location of the prison hospital which survives today as Portchester House, a private residence. Those that died in captivity were often buried in what are now tidal mudflats to the south of the castle, their remains occasionally disturbed by storms.
Today Portchester Castle is used mainly for recreation: the inner section of the castle accommodates displays and exhibits. The castle is a popular venue for school outings, while the sea front is frequented at high tide by anglers in pursuit of flounders and bass.