A castle whose compact design is dictated by the rocky outcrop on which its stands. Roughly rectangular in plan, it has a curtain wall with 8 higher round towers enclosing an outer ward on the W side, and smaller near-square inner ward to the E overlooking the river. Additional defence was provided by barbicans at E and W entrances. Walls are coursed rubble, with freestone dressings of pink sandstone. They are embattled with saddleback copings to the merlons, which also have arrow loops on the towers. The round towers have loops and openings of 2-light mullioned windows, although few of the mullions have survived, and higher round stair turrets. Many features are consistent throughout the building, including freestone fireplaces with raked stone hoods, and window seats.
The main entrance from the town is on the W side. It retains part of a ramp on the N side from the modern Castle Square. The gap over which the drawbridge was lowered has been covered by a timber platform. The entrance arch to the W barbican has a pointed arch with portcullis slots, flanked by round turrets with corbel tables. Inside the gateway are later stone steps to a gate passage, where there is a modern breach in the wall for visitor access, and the springers and draw-bar sockets of another gateway. The W barbican has an almost straight wall with 3 turrets. The town wall is attached to the southernmost turret.
Entrance to the E barbican was from the Water Gate. The outer steps have disappeared, probably lost when Thomas Telford built the suspension bridge in 1822-6, but they are shown on the Buck brothers' 1742 engraving of the castle. Steps inside the barbican have survived, but of the doorway in the barbican wall only the draw-bar sockets have survived. The faceted E barbican wall has 3 turrets similar to the W side.
The castle has 8 towers, of which 6 enclose the outer ward, one at the corners and one half way along each of the N and S walls, and 4 enclose the inner ward, of which 2 (stockhouse and bakehouse towers) are common to both inner and outer wards. In the outer ward the S wall is faceted and the W wall is narrower than the E. Otherwise the whole castle is rectangular in plan. The W wall of the outer ward has a pointed arch, below deep corbels of former machicolations. On the N side of the outer ward, both sections of wall have 2 loops and 2 latrine shafts, including one on the W side contained within a shallow projection and low round turret. Attached to the stockhouse tower, between inner and outer wards, is the town wall. The inner ward N wall has 2 loops and 2 low-level outlets of latrine shafts. On the S side each section of the outer ward has 3 windows, 2 loops to the cellar and latrine shaft at wall-walk level. The bakehouse tower between inner and outer wards is partly rebuilt in snecked stone, with a battered plinth of rock-faced stone, repairs carried out by LNWR in the 1870s of the deliberate breach made in 1655. The inner ward S wall has a doorway at ground-floor level above a battered rubble plinth (the only section not built directly on bedrock). At 1st-floor level are 2 loops, a larger opening centre-R, and a former doorway at the R end. Above 1st-floor level are 3 latrine shafts. The E wall, from the E barbican, has a shoulder-headed doorway, 4 1st-floor windows with stepped lintels, and deep corbelled machicolations, although the embattled parapet has not survived.
In the outer ward, the gate passage has portcullis slots and draw-bar sockets, and a high-level door on the S side, to stone steps up to the wall walk. The inner side of the wall is corbelled out at parapet level. The NW and SW towers form a pair. They each have 2 superimposed newel stairs restored in concrete. Both have fireplaces to 1st and 2nd floors. In addition the SW tower has a domed bread oven at ground floor, and latrine to the 1st floor. The kitchen tower in the centre of the N side of the outer ward has a ruined newel stair. The wall walk is corbelled out around its faceted inner side. The prison tower on the corresponding S side has a dungeon, but otherwise similar details to the other towers, including restored newel stairs and ruined fireplaces, except for a 2nd-floor fireplace with flat stone arch instead of a corbelled lintel.
Remains of buildings can be seen against each of the outer ward walls. Of the guard rooms to the W, flanking the gate passage, and kitchen and stables on the N, only footings have survived. Against the S wall is a long faceted range housing lesser hall and a small chamber in the W facet, great hall in the central facet, passage and chapel in the E facet. At the W end are stone steps leading down to a pointed cellar doorway with continuous chamfer. To its L is a pointed window, its tracery missing but originally 2-light. Further L is a similar former 2-light window to the great hall that retains fragments of bar tracery. In the E facet are the passage doorway, the dressings of which are mostly missing and with modern stone steps, and 2-light chapel window, also with fragments of bar tracery. The chapel has a similar former 3-light E window. Inside, this range has one transverse arch and the springers and haunches of 7 others, all inserted in the mid C14 to support the roof. The lesser hall has a fireplace in its W end wall; the small chamber between halls has a N fireplace; the great hall has a fireplace against the prison tower. Access to the prison tower is from the embrasure of one the S windows of the great hall. The cellar has a dividing wall below the chapel with doorway.
On the E side of the outer ward is a stone-lined well, approximately 91 feet deep. Behind the well was a drawbridge to a small gatehouse at the middle gate between inner and outer wards. The gatehouse is square in plan with narrow loop in the W wall. The middle gate has a doorway with shouldered lintel to each end of its passage, and draw-bar socket.
The other entrance to the inner ward, the E gate, has draw-bar sockets, and a passage giving access to mural stairs to the king's tower and chapel tower. The stockhouse and bakehouse towers are similar to the towers in the outer wards. The bakehouse tower has a domed oven behind the ground-floor fireplace, and restored newel stairs. The stockhouse tower has ruined newel stairs. The NE chapel tower has a restored conical slate roof. From the inner ward is a C19 restored doorway with red sandstone jamb to the ground floor. It also has a passage and doorway above the water gate on the outer (E) side. A mural stair leads to the 1st-floor chapel, where there is also a separate latrine. The rib-vaulted chapel is round with an apsidal sanctuary. The sanctuary has wall shafts and cusped arcading, incorporating sedilia on the S side, below 3 pointed windows with leaded glazing. On the S side is a squint from a small cell. On the N side of the chapel is a deep window seat, which also features a squint to the sanctuary. A restored mural stair leads to the upper chamber. The SW king's tower has a restored newel stair. At 1st-floor level is a small keeled tunnel-vaulted chamber.
Buildings are ranged against the walls of the inner ward, including the king's private apartments. Against the S wall are the 1st-floor king's chamber on the E and king's hall on the W (known as presence chamber and privy chamber respectively in a survey of 1627), with a passage to the bakehouse tower at the W end. This passage has a segmental-pointed arch. Two windows to its L have dressings mostly missing, and further L is a segmental-headed ground-floor doorway and another window. Entrance to the 1st-floor hall is by a doorway above the passage, which has a 2-light cusped square-headed window immediately to its L. Further L are 2 hall windows and a 3rd to the king's chamber, all square-headed with relieving arches, bar-tracery fragments and fragments of sunk spandrels. Next L is the wall over the passage to the E gate (later used as a buttery). From inside the passage the range has a ground-floor doorway with chamfered dressings and springers of a possible cambered arch, and at the L end a 1st-floor doorway with segmental head. The W wall of the hall has 2 1st-floor doorways with segmental heads. Inside, beneath the hall is a ground-floor fireplace to the W wall, and larger former corbelled 1st-floor fireplace in the S wall. One floor-length window reveal in the S wall has a short passage to a latrine. Hall and chamber have one complete and the springers of 3 other C14 stone transverse arches supporting the former roof. In the king's chamber the 1st-floor has floor-length S and E window reveals opening to mural passages to a latrine and the king's tower.
Against the E wall of the inner ward are the passage to the E gate, and what was known in 1627 as the 1st-floor great chamber. The passage on the R has a pointed segmental arch, to the L of which the ground floor has a segmental-headed window and a small window further L. The 1st-floor great chamber was entered by a doorway at the R end over the passage, which has a cambered head. The chamber has one large square-headed W window under a relieving arch. Inside, fireplaces were built into the W walls, with tripartite lintel in the ground floor. The springers and haunches survive of 2 former transverse arches added in the mid C14 to support the roof.