A concentrically planned castle comprising an inner ward, which is square in plan, with high inner curtain wall incorporating gatehouses and towers, an outer ward and an outer curtain wall which is nearly square in plan but has shallow facets to form an octagon. The outer curtain wall faces the moat. The castle is built mainly of coursed local limestone and local sandstone, the latter having been used for dressings and mouldings. Openings have mainly shouldered lintels.
The main entrance was the S side, or Gate Next the Sea. This has a central gateway with tall segmental arch, slots in the soffit for the drawbridge chains, loop above it and machicolations on the parapet. The entrance is flanked by round gatehouse towers which, to the L, is corbelled out over a narrower square base set diagonally, and on the R is corbelled out with a square projecting shooting platform to the front. The towers have loops in both stages, and L-hand (W) tower has a corbelled latrine shaft in the angle with the curtain wall. The shooting platform has partially surviving battlements, and is abutted by the footings of the former town wall, added in the early C15. On the R side of the gatehouse is the dock, where the curtain wall has a doorway for unloading provisions. The dock wall, projecting at R angles further R has a corbelled parapet, a central round tower that incorporated a tidal mill and, at the end, a corbelled shooting platform, perhaps for a trebuchet, with machicolations to the end (S) wall. The E side of the dock wall has loops lighting a mural passage.
The curtain walls have loops at ground level of the outer ward, some blocked, and each facet to the E, W and N sides has higher end and intermediate 2-stage round turrets, and all with a corbelled parapet. The northernmost facet of the W side and most of the northern side were added after 1306 and a break in the building programme. The towers at the NW and NE corners are larger and higher than the other main turrets. On the N side, in the eastern facet, is the N or Llanfaes Gate. This was unfinished in the medieval period and has survived much as it was left. The gateway has a recessed segmental arch at high level, a portcullis slot and a blocked pointed arch forming the main entrance, into which a modern gate has been inserted. To the L and R are irregular walls, square in plan, of the proposed gatehouse towers, the N walls facing the moat never having been built. Later arches were built to span the walls at high level in order to facilitate a wall walk. The NE tower of the outer curtain wall has a corbelled latrine shaft in the angle with the E curtain wall, and in the same stretch of wall is a corbelled shaft retaining a gargoyle. The SE tower also has a corbelled latrine shaft in the angle with the E curtain wall.
In the Gate Next the Sea the passage is arched with 2 murder slots, a loop to either side, and a former doorway at the end, of which draw-bar slots have survived. In the R-hand (E) gatehouse is an irregular-shaped room with garderobe chamber. On its inner (N) side are mural stair leading to the wall walk and to a newel stair to the upper chamber. The upper chamber has a fireplace with missing lintel, and a garderobe. The L-hand (W) gatehouse has an undercroft. Its lower storey was reached by external stone steps against the curtain wall, and retains a garderobe chamber and fireplace, formerly with projecting hood. The upper chamber was reached from the wall walk.
On the inner side facing the outer ward, the outer curtain wall is corbelled out to the upper level, except on the N side where only a short section is corbelled out. To the W of the gatehouse are remains of stone steps to the gatehouse, already mentioned, and stone steps to the wall walk. Further R the loops in the curtain wall are framed by an arcade of pointed arches added in the mid C14. The curtain wall towers have doorways to the lower stage, and were entered from the wall walk in the upper stage. In some places the wall walk is corbelled out and/or stepped down at the entrances to the towers. On the W side, the southernmost facet has a projecting former garderobe, surviving in outline form on the ground and with evidence of a former lean-to stone roof. Just N of the central tower on the W side are the footings of a former closing wall defining the original end of the outer ward before the curtain wall was completed after 1306. Further N in the same stretch of wall are stone steps to the wall walk. The NW corner tower has a doorway with draw-bar socket, passage with garderobe chamber to its L, and a narrow fireplace which formerly had a projecting hood. The upper stage floor was carried on a cross beam, of which large corbels survive, and corbel table that supported joists. In the upper stage details of a former fireplace have been lost.
In the Llanfaes Gate the proposed gatehouses both have doorways with ovolo-moulded surrounds. The L-hand (W) doorway leads to a newel stair. The NE curtain wall tower is similar to the NW tower, with garderobe, fireplaces and corbels supporting the floor of the upper stage. Both facets on the E side have remains of garderobes with stone lean-to roofs, of which the northernmost is better preserved. The SE tower was heated in the upper stage but the fireplace details are lost. In the dock wall, a doorway leads to a corbelled mural passage.
The inner ward is surrounded by higher curtain walls with corbelled parapets. It has S and N gatehouses, and corner and intermediate round towers in the E and W walls. The towers all have battered bases and in the angles with the curtain walls are loops lighting the stairs. The curtain walls have loops lighting a first floor mural passage, and the S and N sides also have shorter passages with loops in the lower storey. The inner curtain wall has a more finely moulded corbel table than the outer curtain wall, and embattlements incorporating arrow loops. The main entrance to the inner ward was by the S Gatehouse. It has an added barbican rectangular in plan. The entrance in the W end wall has a plain pointed arch, of which the voussoirs and jamb are missing on the L side. The S wall has 3 loops and 2 gargoyles, the L-hand poorly preserved, and has a single loop in the E wall. Inside are remains of stone steps against the E wall leading to the parapet. The 2-storey S gatehouse has a 2-centred arch, a pointed window above, retaining only a fragment of its moulded dressings, spanned by a segmental arch with murder slot at high level. The towers to the R and L are rounded and have loops in the lower stage, and square-headed windows in the middle stage.
The SW, W (Middle) and NW towers have similar detail, a loop in the lower stage and blocked 2-light mullioned window in the middle stage. The 3-storey N Gatehouse, although similar in plan and conception to the S Gatehouse, differs in its details. It has a central 2-centred arch and pintles of former double gates. In the middle storey is a narrow square-headed window and in the upper storey a 2-light window with cusped lights and remains of a transom. A high segmental arch, incorporating a murder slot, spans the entrance. The rounded towers have loops in the lower stage. The R-hand (W) has a window opening in the middle storey, of which the dressings are missing, and in the upper storey a single cusped light to the N and remains of a pair of cusped lights, with transom, on the W side. The L-hand (E) tower has a single square-headed window in the middle storey (formerly 2-light but its mullion is missing) and in the upper storey a single cusped light and square-headed window on the E side. The NE and SE towers are similar to the towers on the W side. In the middle of the E curtain wall is the chapel tower, which has 5 pointed windows in the middle storey.
The S gateway has a well-defended passage. The outer doorway has double draw-bar sockets, followed by a portcullis slot, 4 segmental arches between murder slots, loops in each wall, then another portcullis slot and a segmental arch where the position of a doorway is marked by double draw-bar sockets. Beyond, the passage walls were not completed, but near the end is the position of another doorway with draw-bar socket and the base of a portcullis slot.
The gatehouses have a double depth plan, but only the outer (S) half was continued above ground-floor level. The N side has the footings of guard rooms, each with fireplaces and NE and NW round stair turrets, of which the NW retains the base of a newel stair. Above ground floor level the N wall of the surviving building, originally intended as a dividing wall, has doorways in the middle storey. Both gatehouses have first-floor fireplaces, of which the moulded jambs and corbels have survived, but the corbelled hood has been lost.
Architectural refinement was concentrated upon the N gatehouse, which was the principal accommodation block, and the chapel. The S elevation of the N gatehouse has a central segmental arch to the entrance passage. To its R is a square-headed window and to its L are 2 small dressed windows, set unusually high because an external stone stair was originally built against the wall. In the 5-bay middle storey are a doorway at the L end and 4 windows to a first-floor hall. All the openings have 4-centred arches with continuous mouldings, sill band and string course at half height. The R-hand window retains a transom but otherwise no mullions or transoms have survived. Projecting round turrets to the R and L house the stairs, lit by narrow loops. To the N of the R-hand (E) stair tower the side wall of the gatehouse has the segmental stone arch of a former undercroft.
The N gate passage is best described from its outer side, and is similar to the S gate. It has a doorway with double draw-bar sockets, portcullis slot, springers of former arches between murder slots, loops in each wall, another portcullis slot, a pointed doorway with double draw-bar sockets, doorways to rooms on the R and L, and a 3rd portcullis slot. The gatehouses have, in the lower storey, 2 simple unheated rooms. The first-floor hall has pointed rere arches, moulded C14 corbels and plain corbel table supporting the roof, a lateral fireplace formerly with corbelled hood, and a similar fireplace in the E wall (suggesting that the hall was partitioned) of which the dressings are mostly missing. Rooms on the N side of the hall are faceted in each gatehouse, with fireplaces and window seats in both middle and upper storeys. Stair turrets have newels stairs, the upper portion of which is renewed in concrete on the W side.
The Chapel tower has a pointed rubble-stone tunnel vault in the lower storey. In the middle storey is a pointed doorway with 2 orders of hollow moulding, leading to the chapel. Above are 2 corbelled round projections in the wall walk. The chapel doorway opens to a small tunnel-vaulted lobby. Entrance to the chapel itself is through double cusped doorways, which form part of a blind arcade of cusped arches with trefoiled spandrels, 3 per bay, to the 2-bay chapel. The chapel has a polygonal apse and rib vault on polygonal wall shafts. The W side, which incorporates the entrance, also has small lancet openings within the arcading that look out to the mural passage. Windows are set high, above the arcading. The W bay has blind windows, into which small windows were built that allowed proceedings to be viewed from small chambers contained within the wall on the N and S sides of the chapel, reached from the mural passage and provided with benches.
The SW, NW, NE, SE and the Middle tower are built to a standard form, with round lower-storey rooms, octagonal above. They incorporate newel stairs, of which the NW has mostly collapsed, and the SW is rebuilt in concrete at the upper level. The lower storey, which has a floor level lower than the passage from the inner ward, was possibly used as a prison and has a single inclined vent but no windows. Upper floors were supported on diaphragm arches, which have survived supporting the middle storeys of the Middle and SE towers, whereas the SW and NE towers retain only the springers of former arches, and the NE tower has a diaphragm arch supporting the upper storey. In the middle storey of each tower is the remains of a fireplace with corbelled hood.
Each section of curtain wall contains a central latrine shaft, with mural passages at first-floor level incorporating back-to-back garderobes. The N and S walls also have short mural passages in the lower storey to single garderobes in each section of wall. Mural passages have corbelled roofs. The S side is different as it has tunnel-vaulted lobbies adjacent to the towers, between which are short sections of corbelled passage with garderobes. The wall walk also incorporates back-to-back latrines, in this case reached down stone steps.
There is evidence of buildings within the inner ward. Footings survive of a building constructed against the E end of the N wall. In the curtain wall are 2 fireplaces, formerly with corbelled hoods, to a first-floor hall. On the S side of the chapel tower is the stub wall of a larger building. On the N side of the W curtain wall are the moulded jambs of a former kitchen fireplace, and adjacent to it against the N wall is the base of a bake oven. On the E side of the S curtain wall the wall is plastered to 2-storey height.