The castle is built upon the principle of concentric fortifications, with an inner ward and middle ward surrounded by a moat, dams, two great lakes and outer defences. The inner and middle wards are rectangular in plan with round angle towers and large twin-towered gatehouses to the E and W. An outer gatehouse to the E forms the main entrance and is flanked by fortifications along the N and S dams. The massive battlemented walls are mainly constructed of local pennant sandstone, with some red sandstone and blue lias limestone. The doorways have shallow pointed arches, and the original structures have trefoil-headed lancet windows and arrow slits with small circles at the base. Dressings are of Sutton stone and the style of mouldings can be related to the chronological development of the castle. The only exception is Despenser's great hall where the dressings are of Jurassic limestone from Bristol.
The inner ward is an open courtyard with a well in the centre. The great hall and chambers are to the S, gatehouses to the E and W, and a wall-walk to the N. The great hall is the most highly embellished of all the structures in the castle. The exterior wall, facing into the courtyard, has been refaced in ashlar. It contains 2 pairs of tall 2-light windows with pointed ogee heads and a doorway to the L. Inside the hall, the arched brace roof is late C19. The trusses are supported on the early C14 corbels which consist of 3 long filleted shafts, with bell capitals and stops depicting 3 heads (the King and his courtiers). The corbels and recesses of an earlier roof structure also survive: the corbels are short with roll mouldings. The window reveals are decorated with C14 ball flower ornament. Between the 2 pairs of windows is a large fireplace but the dressings are missing. The E, S, and N walls contain blocked openings showing that the hall was altered in the C13 - 14. To the S of the hall at a high level is a covered vaulted passage (known as the Braose passage) which was originally open. A large infilled archway in the E wall now contains a doorway into a late C20 kitchen. A doorway to its L is original and leads to a room with 2 large fireplaces. To the W, a doorway with broach stops to the jambs leads into 2 large ruined chambers, also much altered: the chambers are irregular in shape, over 2 storeys high, roofless and contain the remains of domestic fireplaces. Tall windows face into the courtyard, mainly ruined, one with the remains of a traceried head in a grey stone dressing.
The E gatehouse forms the main entrance to the inner ward and is the most strongly defended structure in the castle. It is a 3-storey block flanked by projecting round towers (the outer face rebuilt in early C20). Central pointed-arched entrance with chamfered dressed jambs. The towers have arrow slits at 5 levels. Small projections to N and S housing doorways leading to wall-walks. Above and flanking the inner doorway, and also to the exterior, are small trefoil-headed lancets. Two large windows to interior 2nd floor, each with 2 trefoiled lights and a quatrefoil. The upper floors of the gatehouse are thought to have been the private apartments of Gilbert de Clare's constable. The 2nd floor is continuous across the gatehouse and has a large fireplace with raked hood on corbels against the W wall. The floor does not survive. Side chamber with quadripartite vault and heavy chamfered ribs.
The inner W gatehouse is smaller than the inner E gatehouse but in similar style. It is 2-storey with projecting round towers to the exterior and a flat wall to the interior (E) side. The entrance is flanked by 2 low doorways to the interior side, most of the dressings C20, which provide access to the towers. Two large ruined windows under relieving arches to upper storey. The towers have arrow slits at 4 levels. The lower chambers are vaulted, with heavy ribs supported on bell-shaped capitals. The 1st floor of the gatehouse is open, and has concrete beams on concrete corbels in place of the ceiling. Remains of stairs in N tower.
Of the angle-towers of the inner ward, the NW and SW towers stand full-height but were reconstructed in the early C20. They have lancets and arrow slits as in the gatehouses. The upper 2 floors of each tower contained accommodation, whilst the lower floor was for defence purposes and storage. The NW tower has mural stairs to the R of the entrance and a garderobe to the L. The window openings have wide internal splays, shallow pointed heads and occasional window seats. The 2nd floor has a large ornate fireplace with square head and raked hood, partly reconstructed. The 1st floor fireplace is plainer with a flat lintel. The SW tower has lost its floors but an opening leads to the Braose gallery which runs behind the great hall and on to the SE tower. The SE tower is ruined (though part stands full height), and leans outwards at an angle of about 10°. It was not restored by the Bute family so is important for retaining original detail. Very little survives of the NE tower: part of a staircase, the bottom of a window opening, and the base of a door jamb with pyramid stop. A wall-walk runs along the N side of the inner ward and is battlemented with arrow loops. Timber hoarding was reconstructed in the 1990s. Small postern gate in outside of wall at low level.
The curtain wall of the middle ward is lower than that of the inner ward. It has bastions in place of angle towers, on which there is a wall-walk. There are gatehouses to the E and W, smaller but similar in style to those of the inner ward. The 2-storey E gatehouse is ruined. The front elevation with projecting round towers survives, but the rear wall (added later) is ruined. Adjoining to the N is a square projection allowing access to the wall-walk; lancets appear to have replaced arrow slits. The W gatehouse has original projecting round towers with arrow slits. The entrance, containing double doors, is within a flat-headed surround, the cornice supported on corbels. Above is a high relieving arch over 2 windows with shallow pointed heads. The internal face of the gatehouse (also later) is ruined, but the towers retain parts of windows at ground floor level, and a fireplace in one of the upper chambers. (It is said that this area was remodelled in the C16 by the Earl of Pembroke for his Manorial Court.) The wall-walk flanking the gatehouse has been incorporated within later buildings, particularly to the N. The S side of the middle ward was blocked, at the outbreak of war between Llywelyn ap Gruffudd and Edward I, by a large square kitchen annexe which butts against the great hall. To its W is a large ruined round tower with S window, beyond which is a postern, a narrow rectangular block with a low doorway entered from the lake and 2 further openings above. Against the E side of the kitchen block is a circular oven, possibly the remains of a brewhouse. South of the E gatehouse in the middle ward is the remains of a very large structure, probably a storeroom. The basement survives with the raked sills of 2 windows.
A moat surrounds the middle ward to the E, N and W, wooden bridges on stone abutments spanning it to the E and W. There is a small postern gate at a low level in the N wall, providing access from the moat. To the S is the S lake. To the N of the moat is an earthwork dam, beyond which is the N lake. W of the moat is a hornwork of irregular shape, an earthwork surrounded by a low masonry wall. It is an island, being linked to the ground to the W and the earthwork dam by further wooden bridges. Both lakes are retained to the E by masonry dams known as the S and N dam platforms. In front of the dams is the outer E moat. Much of the S dam platform belongs to the original phase of the structure, whilst the outer main gatehouse and N dam platform are slightly later, but pre-date the death of Llywelyn ap Gruffudd in 1282.
The 2-storey outer main gatehouse, partially rebuilt in the early C20, has twin polygonal towers on square bases with pyramidal spurs. They flank the pointed arched entrance which is under a high blind arch. The rear wall is flat, and there is a higher block to the N which provides access to the N dam platform, resulting in an L-shaped plan. The form of the arrow loops is new: cross-shaped with the arms terminating in small circles. On the internal wall above the gateway is a window with 2 trefoil-headed lights and a large transom. The interior of the outer main gatehouse was used for accommodation: the N block contains a spiral staircase and possibly servants quarters; the 1st floor chamber has a fireplace with recess and what appears to be a squint. The 2nd floor chamber no longer has floor boards but there is a door leading to the main chamber over the gatehouse. This has a large fireplace to the W and pointed arched doorways with pyramid stops.
The N dam is in the same style as the outer main gatehouse: 3 small polygonal towers on square bases and cross-shaped arrow loops to the front face, internally with wide splayed reveals. The dam has been raised and is terminated by the N gatehouse (the cavalry entrance). It is in the same style but has lost most of its arches and dressings.
The S dam is wider than the N dam and in a different style, being part of the original phase. Its substantial outer wall is retained by closely-spaced thin square buttresses with splayed bases; the recesses between are concave. Much fabric reconstructed in the early C20, but these areas differentiated by red tile in the infill. Internally, a small turret leads onto the wall-walk, which is terminated at the S end by a small square tower, Felton's tower. This overlies the former culvert which drained water from the S lake to the outer moat (now realigned to the N). The outer wall curves round to the SW where there is a further small rounded tower, the internal wall of which is later, with moulded jambs and pyramid stops to the doorway, and a 2-light window to the upper storey. Further round, and facing W is the S gatehouse which led into the town. It was rebuilt or repaired in the late C13, but is mainly C20, with cross-shaped arrow loops to the exterior round towers. High blind arch above entrance. Flat internal wall with lancets plus a larger window with transom above the gateway. On the S dam platform are the remains of the former corn mill: the head and tail races are still visible but the wheel pit (for an overshot wheel) has been infilled. A room to the N was used for preparing the corn and stands to less than 1 storey high, with the raked sill of a former window.