Portchester Castle originated as the “Saxon Shore” fort of Portus Adurni. It is the best preserved of the chain of Roman forts erected along the southeast coast in the late third century AD. The reason for their construction is still debated. Defense against Germanic raiders is the for the most part the likely explanation and they were without a doubt used for that purpose in the following century.
The fort survives in such good condition for the reason that it was in continued use after the Roman departure, first sheltering a Saxon burgh and then becoming an outer bailey of the medieval castle. Henry I restored the fort’s crumbling walls, built the present gate towers, called Land Gate and Water Gate, and created a rectangular inner bailey in the northwest corner of the fort. It is defended by the Roman wall on two sides, and on the other two by a stone curtain with a projecting gate tower.
Another square tower is positioned diagonally at the vulnerable southeast corner. Portchester is thus an early example of a castle employing flanking towers, perhaps inspired by the Roman bastions. The inner curtain is overshadowed by the lofty square keep, which has displaced some of the Roman wall at the northwest corner of the castle. It is the product of two phases, as shown by the pilaster buttresses which disappear two-thirds of the way up;
Evidently, Henry I’s keep comprised only two stories plus roof space. Its heightening to four stories is ascribed to the Manduits, who held the castle prior to Henry II’s seizure. The keep is divided by a cross wall and entered via a fore building, which is another addition of the second phase. Originally, the entrance was at first floor level, the ground floor doorway being a later insertion.