Masonry castle, now roofless; of limestone and granite construction faced with roughly-squared and coursed blocks; sandstone dressings to the openings; rock foundations. The castle is of the concentric type and occupies a tight, restricted site on a natural rock outcrop. The main defences consist of a four round towers placed at the corners of a roughly rectangular high-walled inner ward, with a lower-walled outer ward forming a narrow secondary line of defence. The latter broadly follows the line of the inner ward, though its walls have been reduced in height.
In the centre of the inner ward's eastern (town-facing) wall is a large square gatehouse block. This consists of a pair of high D-shaped drum towers to the front, and a pair of narrower stair towers to the rear, the latter rising up two stages above the inner ward's corner towers and one stage above the paired front towers. The hall, chapel, service and storage buildings were ranged around the N, S and W sides, with the well located against the N wall. Of these buildings, only the walls of the gabled chapel stand to any height; the remainder is reduced to foundation level. The gatehouse housed the principal rooms, arranged as lavishly-appointed suites, one on each of the upper floors. The first-floor apartment probably served as the constable's quarters, whilst the upper floor rooms, of rather finer quality of detail, would have been reserved for important visitors such as the King and his circle. Access to these apartments was from the inner ward. Here an external stone stair, arranged in three straight flights, leads to a round-arched entrance off-centre to the L on the gatehouse's 3-window inner elevation. The windows to the upper apartments have finely-cut hollow-chamfered jambs and segmental heads, formerly with Decorated tracery lights, typical of the Edwardian Court Style. Various fragmentary fireplaces survive, mostly with stone hoods with decorative corbels. The gatehouse was conceived as an independently defensible unit, and correspondingly could withstand assault from the inner ward, should this be taken. The main approach is in the form of a tunnel entrance between the front towers. This leads through to the inner ward via a system of defences including portcullises, gates and murderholes. The portcullises were operated from the first floor and were therefore under the constable's direct control.
The outer ward has a postern gate, comprising a gateway between small turrets, corbelled-out from the wall; a complex bridge system with further gates and a large drawbridge no longer survives. On the N side is a postern gate with small D-shaped drum towers. This gives access to 'Castle Rock', the rocky plateau which provides the site. A wall runs north-eastwards and curves around and down to protect the rock on this side. At the foot of the rock, at the north-western point, is the 'Gate-Next-the-Sea', where supplies were landed by ship. This gateway had its own drawbridge and portcullis system and was further covered by two rock-cut engine platforms above. It was linked to the main castle by a walled and defended track, known as the 'Way From the Sea', which winds its way up along the E and S sides of the rock.