Oystermouth Castle is an impressive Medieval castle, occupying a commanding hill-top position overlooking Swansea Bay 6.5km south-east of Swansea. The castle was constructed over a series of at least six phases between the 12th century and late medieval period, with the final phase enclosing an area of approximately 1906 square metres.

The early history of Oystermouth is not recorded, though it is probable that the castle was founded by William de Londres I around AD 1107 when Henry Beaumont conquered Gower. This primary phase of the castle is likely to have to have been an earthwork and timber construction, and was probably destroyed by the Welsh incursion into Gower in 1136. It was restored between 1138 and 1141 by Maurice de Londres, who strengthened the castle by adding a rectangular stone Keep (Period I). The first historical record of the castle appears in 1215 when Maelgwn ap Rhys and Rhys Ieuanc invaded Gower and marched on Oystermouth from Swansea, burning the castle along with the adjacent settlement. Period II of masonry construction, a two-storeyed addition set against and integrated with the northern wall of the Keep, was probably carried out by John de Braose after he recovered Gower from the Welsh in 1220. The north-west block and earliest elements of the west range (Period III) are likely to date to a phase of strengthening the castle immediately following the Welsh attack on Gower in 1256. Construction of the Gatehouse and curtain-walls (Period IV) are likely attributable to William de Braose II (1241-90).

The Gatehouse is of an unusual and undeveloped form, and the lack of arrow-slits commonly adopted elsewhere in Wales by 1270 suggest that Period IV of construction was carried out between 1256 and 1266 not long after the completion of Period III. By 1284 William appears to have regarded Oystermouth as his most impressive residence as he entertained Edward I there on the 10th and 11th December, rather than at his caput at Swansea. In 1287 there was another violent Welsh rebellion and Oystermouth Castle was once again burned in June. The Chapel-Block (Period V) is the tallest and most substantial structure on the site. It has sometimes been misinterpreted as a 12th-century Norman Keep, but demonstrably post-dates the Keep on the central knoll and its architectural details are stylistically work of the early 14th century. Persistent tradition claims that Alina de Braose (1327-31) added the chapel; however whilst the traceried windows are not inconsistent with this, only the prison is recorded in her time. There are traces of wall paintings in the southern recess of the chapel and on an area of plaster beneath the vault. These consist of a repetitive diamond pattern containing flowers and coats of arms. Traces of pigment have also been noted in the south-east angle of the recess. In the later Middle Ages (Period VI), two large service buildings were added against the curtain-walls in the southern court, and latrine turrets were inserted in the southern curtain-walls.

In the post-Medieval period the castle was held by the Earls of Worcester (later Dukes of Beaufort), apart from a brief period when it was held by Oliver Cromwell. There is no evidence for any military action or further building works in the post-Medieval period, though clearance and restoration works were undertaken in the 1840s and 1870s by the antiquary George Grant Francis.

In 2011 Oystermouth Castle was reopened to the public following a £1million refurbishment, funded by Cadw and the European Regional Development Fund. This included the addition of a £10m high glass bridge to allow the public to access the second-storey chapel. As part of these works archaeological and survey investigations were carried out by Gwent Glamorgan Archaeological Trust in partnership with the Friends of Oystermouth Castle, Swansea City Council and Swansea University, including a comprehensive volunteer training programme.

On 30th July 2014 aerial photography carried out by the Royal Commission identified a partial rectangular parch mark to the east of the chapel block suggesting the foundations of a substantial stone building may lie undiscovered outside the curtain-walls.

Painted features include flowers and coats of arms from 1697. Other coats of arms in a recess have aslo been recorded.

Sources include:
CADW listed buildings database
Gwent Glamorgan Archaeological Trust Report 1994/23
Gwent Glamorgan Archaeological Trust Interim Report 2009/085
RCAHMW 2000. Inventory of Ancient Monuments in Glamorgan Volume III Part 1b - Medieval Secular Monuments: 245 ? 272
Richard Suggett, Painted Temples: Wallpaintings and Rood-screens in Welsh Churches, 1200–1800, (RCAHMW 2021)