Portchester Castle is a medieval fortress that was developed within the walls of the Roman Saxon Shore fort of Portus Adurni at Portchester, to the east of Fareham in Hampshire.
The keep was probably built in the late 11th century as a baronial castle and Portchester was taken under royal control in 1154. The monarchy controlled the castle for several centuries and it was a favoured hunting lodge of King John. It was besieged and captured by the French in 1216 before permanently returning to English control shortly thereafter.
Occupying a commanding position at the head of Portsmouth Harbour, in the medieval period Portchester was an important port. The castle saw the embarkation for several campaigns to France led by England's kings. In anticipation of a French invasion during the first quarter of the 14th century, Edward II spent £1,100 repairing and reinforcing Portchester Castle. A plot to overthrow Henry V was discovered and the culprits apprehended at Portchester; this event features in Shakespeare's play, Henry V. Later in its history, the castle was used as a prison.
Today Portchester Castle is a Scheduled Ancient Monument, and a Grade I listed building.