Portchester Castle began as a square Roman fort, approximately 200 yards wide, surrounded by high walls with 4 D-shaped bastions spaced along each side and one at each angle. Of the 20 bastions, 14 survive today, while the walls, of flint with bonding courses of brick and stone, stand to a height of approximately 20 feet. The four walls face more or less N, S, E and W, with main entrances in the centres of the W and E walls (the latter a water gate), and narrower posterns at N and S. The entire enclosure occupies more than 8 acres and was built in the late 3rd century. It is sited on a promontory at the N end of Portsmouth Harbour, with the sea guarding the E and S sides and ditches protecting the W and N. In the NW corner of this great enclosure is the medieval castle, begun in the 12thc., and in the SE quadrant, as far away as it can be within the confines of the fort, stands the Church of St Mary, formerly a Priory.

The castle has a square keep which replaces the Roman bastion at the NW angle, and a small inner bailey constructed by building walls at the E and S, surrounded by a moat. The keep is of 4 storeys, and was built of Caen and Quarr ashlar. The ground floor is an undercroft and the keep was entered at 1st-floor level by a completely plain doorway in the E wall approached by an external stair covered by a forebuilding, now ruined. The second floor contained the roof of the original, early-12thc keep, and around 1150 the roof was raised making it useable. A third floor was added in the early 14thc. The inner bailey is rectangular with medieval ranges on all four sides surrounding a central space. Entry is through a gatehouse towards the E of the S range with a wooden bridge over the 12thc moat. The 12thc gatehouse was an open sided tower, which still remains, and this was extended east ward in stages. In the 1320s a vaulted porch closed by a portcullis was added to the outside, then in the 1380s it was extended outwards again, and finally a two-tower facade was added c.1600.

The short secton of the S range to the E of the gatehouse was remodelled by Sir Thomas Cornwallis in the early-17thc. as a kitchen, and is now occupied by the English Heritage shop and ticket office. A few 12thc. carved stones have been set over the N doorway and window, and are described in this report. The long building to the W of the south range is the Great Hall built by Richard II, with an undercroft below and a kitchen at the E end. The W end was the high end of the hall, and Richard II's Great Chamber with its undercroft occupies the W range, abutting the 12thc keep at its N end. On the E side of the Great Chamber, at the N end is the 2-storey Exchequer Chamber, also added by Richard II, which partly obscures a 12thc window on the 1st floor of the keep.

The N range of the inner bailey contained the Constable's residence, with a hall above an undercroft, but these only survive as low walls. In the NE corner of the bailey is Ashton's Tower, built by Sir Robert of Ashton who was Constable between 1376 and 1381. Finally the E range is defined by a ruinous 16th-17thc wall built by Sir Thomas Cornwallis the Constable under Elizabeth I, who remodelled what had originally been a service range and was later partly absorbed into the Constable's residence. The post-12thc. work of the castle contains a few reset Romanesque stones, described below. In-situ 12thc. sculpture is confined to a pair of windows on the 1st floor of the S wall of the keep, and to a billet stringcourse around the exterior of the keep between the ground and 1st floors.

At the time of the Domesday Survey, William Mauduit held the manor of Portchester, which had been held before the Conquest by 3 freemen of the Confessor as 3 manors. A hall was mentioned in 1086, but the castle was not yet there. The manor passed from William to his elder son Robert, who died in the White Ship disaster of 1120, and the castle and its lands reverted to the crown. Robert's heir was his daughter, married to William Pont de l'Arche, the Sheriff of Hampshire, who probably rebuilt the castle, perhaps under instructions from Henry I.

During the reigns of Henry II, Richard, John and Henry III the castle was frequently visited by the Court as a staging point on journeys to the Continent for peaceful or warlike motives, and in Edward II's reign it was kept fully equipped in expectation of an invasion from France. As for work on the castle, The keep was built up to first-floor level by the s, and was raised by another 2 storeys by the 1150s. There is evidence for rebuilding of the E and W gates in 1172 and for 12thc work in the bailey. Also in the 12thc the main E and W gateways were rebuilt. There is evidence of repairs to a royal residence separate from the keep, presumably in the inner bailey, in 1183. King John built a new chamber and wardrobe at the castle in 1211. The castle mill was repaired in the reign of Edward I, and his son Edward II, set the buildings in good order, renewed buildings in the inner bailey raised the keep by another storey and again repaired the W and E gates. Edward III also repaired ruinous buildings at the castle and repaired the Watergate yet again, as well as mending breaches in the Roman south wall. The most significant surviving later Medieval work was carried out for Richard II by his Master Mason Walter Walton between 1396 and 1399. He remodelled the royal apartments, the keep and the W or Landgate. The castle was still in use in 1563, when it became a military hospital for men wounded in the French wars, and in 1583 it was refortified in expectation of a Spanish invasion. The last Constable of the castle, Sir Thomas Cornwallis, who rebuilt the E range. He died in 1618 and was buried in St Mary's, Portchester. In 1632 the castle was bought from Charles I by Sir William Uvedale and it remained in the ownership of that family, and their heirs the Thistlethwaites, until 1926. For most of that period it was commandeered by the army, who used the castle as a prisoner of war camp during various wars between 1665 and 1814. Thereafter it stood as a ruin until 1926 when the Thistlethwaite owners placed it in the guardianship of the Office of Works, and in 1984 it passed into the possession of English Heritage. A fuller account of medieval royal works at the castle may be found in VCH (151-58) and the Historic England Research Record.

Cunliffe (1985), 78 compares the original form of the 1st-floor keep windows with those in the Salle d'Echiquier in Caen Castle. The arches seen in the Great Hall undercroft have been reconstructed in Cunliffe (1985), 98, as the in-situ interior decoration of that wall as it was in the 12thc. This shows a pair of double bays with a continuous chevron order inside a moulded order carried on capitals, but the authors are clearly uncomfortable with such an elaborate arrangement of blind arcading in such a narrow space, and in what one would expect to be an undercroft. The present author is thus inclined to treat the structure as an assemblage of pieces reset from elsewhere on the site.

This raises another issue, which is that some of the carved stones reset around the castle may have been reused from the conventual buildings of the priory church, when they were no longer needed.