Speculated that Portchester Castle may have been built on the site of an earlier Roman fortification (dating back to the 3rd Century), it was constructed possibly as early as the 11th Century (during the reign of William The Conqueror). The importance of Portchester Harbor was well known, so the need for a fortification to protect the Harbor's entrance was of need. Fearing early 14th Century French invasions from The Hundred Years War, King Edward II of England spent vast amounts of funds to turn Porchester Castle into a proper fortress. Having a large Outer or Open Bailey (also known as a Base Court), at the castles direct front entrance. Originally an Augustine Church was built at the far end of the Open Court. Obviously more interior structures were planned (but never came into being). Henry II had personal apartments built at Portchester, and stayed there often. It was here that he may have even planned the "removal" of friend (and rival), Archbishop Thomas Becket from office.
King John also stayed at Portchester on many occasions. He even planned numerous invasion on Normandy, France from there. Heavily garrisoned by troops during The Hundred Years War, the castle became a center for the local economy (as did the Harbor). King Henry VIII and his wife Anne Boleyn also visited. In 1603, his daughter - Queen Elizabeth I held Court there many times. Portchester saw no active service during The English Civil War (although a detachment of Parliamentary Dragoons were stationed there). During The Second Dutch-Anglo War (1665-67), some 550 prisoners were kept at the castle. Some attempted escape, and set fire to most of the interior structures. It was never repaired and stayed dormant for 40 years. In the 19th Century, Portchester again served as a prison (this time for Napoleonic War prisoners). It's unusual design features two defense Towers (one larger located "landside" and the other - smaller one, facing the Open Court). The Chapel/Church still stands at the far end of the Court expanse (facing the water). The castle is currently open to the Public.