When built, initially as a timber structure at the start of the 12th century, Swansea Castle overlooked a strategic crossing point of the River Tawe, 300 feet east, and for several centuries it played an important role in the governance of this part of south Wales, converted to stone in the 13th century and in regular use until the 1500s. Since then it has gradually been reduced in size as parts became ruined or were demolished to make room for new developments, then in Victorian times the course of the river was moved a few hundred feet further east, and the intervening land developed, leaving the fragmentary castle remains surrounded on all sides by modern buildings, unrecognisable from the original setting.
The surviving sections are still quite substantial however - the main part is over 50 feet high, containing several rooms, on three levels, topped by an elegant arcade of round-headed arches. This is the south block, which is linked on the east side by a short section of curtain wall to part of the north block, a much smaller structure latterly used as a prison, and significantly altered from its original state.
Although parts of the interior of both blocks are still structurally sound, all are closed and fenced off, so only the exterior of the ruin can be viewed. The site has a few interpretive notices, but seems to be not much visited, and is not promoted as a major attraction of the city. The front adjoins a road (Castle Street, the B4489), beyond which is a plaza with fountain and flowing water, this occupying the western part of the of the original castle site. There are plenty of parking options nearby, and the ruins can be seen in just ten minutes or so.
The castle at Swansea was established in 1107 by Henry de Beaumont, 1st Earl of Warwick, shortly after he was created Lord of Gower by Henry I, the reward for several years of faithful service. This initial structure, made entirely of timber, was attacked by the Welsh in 1116 but not greatly damaged, then besieged for a second time by Rhys ap Gruffydd in 1192, again surviving largely intact. The castle, a few hundred feet north of the current ruin, consisted of an approximately oval-shaped embankment surrounding the wooden fort, augmented to the north, west and south by outer defences, with the River Tawe providing protection from the east. Work on the stone replacement, subsequently known as the New Castle, began around 1220 and took several decades to complete: it is situated at the southeast corner of the original outer bailey, and probably consisted of a square enclosure with ranges on all sides, and a keep or tower at the centre, with the main entrance to the west. The castle was modified and improved early in the 14th century when in possession of the de Mowbray family, though its peak was relatively short-lived, and the place was largely disused by the early 1500s, and ruinous in the next century. After this time parts were converted for other uses, and some sections were removed. The river was diverted in the 1840s, and the original channel converted into a dock, then this was drained and filled in the mid 20th century.
The smaller of the two remaining components of Swansea Castle is the north block, a rectangular tower occupying the northeast corner. At ground level are three vaulted chambers, with arrowslits, below several rooms that were added in the late 1700s, when the structure was used as a debtor's prison. The south block is much more spectacular, especially its south wall, which is topped by an elegant arcade of arches, probably part of the early 14th century improvements. The design is continued as blind arches across two sides of the southeast corner tower. Below the arcade are two tall windows from the first floor great hall (roofless, but retaining its floor), then lower still are three especially tall arrowslits, illuminating vaulted chambers at ground level. On top of the arcade are fragments of a further wall, containing both arrowslits and windows. The southeast tower is a substantial, rectangular structure with a basement, a first floor solar (living quarters) and chambers above, surviving nearly to its full height. The southeast corner is formed by a circular garderobe tower, also intact up to roof level, adjoining a circular turret.