Portchester was built originally by the Romans during the reign of Emperor Carausius (285-293) as a defensive for that was part of many other ‘Saxon Shore Forts’ that had been built along the south and east coasts of Britain to protect themselves against Saxon raids. Portchester itself was built in such a place that it could protect the area and be vigilant against enemy attacks, but also where it could defend the harbour. This made Portchester a perfect anchorage for Rome’s navy and the fort had an excellent vantage point over all of this.
They also built other defensive structures such as a curtain wall and bastions around the outside. The original fort had a curtain wall built around it, made primarily from timber that was just a straight rectangle around the buildings inside. The Romans also built the Landgate where people would enter the fort from the mainland. It was originally just a reinforced gate with a bridge and provided basic protection from attacks. After the Romans left Britain, Saxon settlers inhabited the fort, building their houses within the Roman walls.
Although the Saxons did not build much of their own from scratch, they did undertake lots of reconstruction work and rebuilding, especially on a stone tower and the original Roman Watergate, improvements that are predominantly visible today. The Saxons made these repairs and additions to the castle because they were afraid of attack from people like the Vikings who were fighting over the land in Britain. After the Norman Conquest in 1066, the Normans built up the Roman fort and made improvements to the walls.
This created their inner bailey in which they built the keep. Unlike most castles, the keep was built in the corner of the castle and was incorporated with the Roman walls that were now being used as outer bailey defences. The keep was used mostly for fortified accommodation. The Normans also built the priory and the original gatehouse. The gatehouse was extended and refortified several times to protect itself against new technologies that were now arriving in castle attack.
It’s not unfair to say that the Normans made the most progress into what we see today because they need to use the castle as a weapon of power and intimidation against the locals who had been recently invaded. However, in the middle of the 12th century, the castle was passed over to the crown and Henry I saw it as an excellent place to store the treasury bullion. At the same time, Portsmouth was founded a small way away and it started to rapidly develop as a dockyard.
Because of this, although Portchester was still an important strategic royal castle, it started to become rather overshadowed and isolated. During the Medieval period, the castle began to decline slightly in importance, but was still vital as a strategic area of defence. The Hundred Years War was a time were the castle got some much needed improvement and refurbishment. The defences of the gatehouse, Watergate and Landgate were all improved and the buildings of the inner bailey were also extended.
This all came about because the French were now posing a serious threat after they attacked and burned Portsmouth. Then, in 1367, Sir Robert of Ashton became constable of Portchester Castle and built Ashton’s tower, a fortified tower that contained private rooms, built in the corner of the inner bailey. The tower itself was built to modernise the castle to new weaponry because it had gun slits and other fortifications to accommodate new weapons. By this point, Portchester Castle had reached the end of its life as a protector of the harbour.
When war with the French ended in 1396, Richard II built a palace at Portchester and now it seemed it was being refurbished only to improve comfort. This was because the defences of the castle were now beyond any more improvement and it was now of no use a harbour defence structure. Then in the Tudor period, Henry VII decided that a royal dockyard should be built in Portsmouth because it was a much better area to place ships and fortifications. Portchester harbour was now too small and had silted up, making it impractical for the new large ships being brought in for docking.
By now, the castle was spending periods of time uninhabited and nearly no money was being spent on refurbishment or reconstruction. In the 17th Century, Portchester was inhabited again for a while as it was being used as a prison, first during the Dutch War of 1665-7 and then in the 18th century when it was housing Spanish and French prisoners. It took even more prisoners during the Napoleonic Wars and evidence of its use as a prison can still be seen today such as: within the keep, hooks where hammocks in which prisoners slept in and barracks in the outer bailey that housed soldiers.
Portchester Castle was now no longer useful as a protector of the harbour and was technologically out of date, despite some additions like Ashton’s tower, rendering it useless against newer gunpowder weapons like cannon in the 16th Century. It was also being overshadowed by the better naval base at Portsmouth, meaning the castle’s strategic importance was now declining. In the 19th Century, Portchester Castle finally became a ruin and was later taken over by English Heritage.