White Castle (Welsh: Castell Gwyn), also known historically as Llantilio Castle, is a ruined castle near the village of Llantilio Crossenny in Monmouthshire, Wales. The fortification was established by the Normans in the wake of the invasion of England in 1066, to protect the route from Wales to Hereford. Possibly commissioned by William fitz Osbern, the Earl of Hereford, it comprised three large earthworks with timber defences. In 1135, a major Welsh revolt took place and in response King Stephen brought together White Castle and its sister fortifications of Grosmont and Skenfrith to form a lordship known as the "Three Castles", which continued to play a role in defending the region from Welsh attack for several centuries.
King John gave the castle to a powerful royal official, Hubert de Burgh, in 1201. Over the next few decades, it passed back and forth between several owners, as Hubert, the rival de Braose family, and the Crown took control of the property. During this period, White Castle was substantially rebuilt, with stone curtain walls, mural towers and gatehouses, forming what the historian Paul Remfry considers to be "a masterpiece of military engineering". In 1267 it was granted to Edmund, the Earl of Lancaster, and remained in the hands of the earldom, and later duchy, of Lancaster until 1825.
Edward I's conquest of Wales in 1282 removed much of White Castle's military utility, and by the 16th century it had fallen into disuse and ruin. The castle was placed into the care of the state in 1922, and is now managed by Cadw, the Welsh heritage agency.
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