Chepstow Castle. Chepstow Castle at Chepstow, Monmouthshire, Wales is the oldest surviving post-Roman stone fortification in Britain. Located above cliffs on the River Wye, construction began in 1067 under the instruction of the Norman Lord William FitzOsbern. Originally known as Striguil, it was the southernmost of a chain of castles built in the Welsh Marches, and with its attached lordship took the name of the adjoining market town in about the 14th century. In the 12th century the castle was used in the conquest of Gwent, the first independent Welsh kingdom to be conquered by the Normans. It was subsequently held by two of the most powerful Anglo-Norman magnates of medieval England, William Marshal and Richard de Clare. However, by the 16th century its military importance had waned and parts of its structure were converted into domestic ranges. Although re-garrisoned during and after the English Civil War, by the 1700s it had fallen into decay. With the later growth of tourism, the castle became a popular visitor destination. The ruins were Grade I listed on 6 December 1950. Chepstow Castle is situated on a narrow ridge between the limestone river cliff and a valley, known locally as the Dell, on its landward side. Its full extent is best appreciated from the opposite bank of the River Wye. The castle has four baileys, added in turn through its history. Despite this, it is not a defensively strong castle, having neither a strong keep nor a concentric layout. The multiple baileys instead show its construction history, which is generally considered in four major phases. The first serious architectural study of Chepstow began in 1904 and the canonical description was long considered to be by Perks in 1955. Recent studies have revised the details of these phases, but still maintain the same broad structure. The speed with which William the Conqueror committed to the creation of a castle at Chepstow is testament to its strategic importance. There is no evidence for a settlement there of any size before the Norman invasion of Wales, although it is possible that the castle site itself may have previously been a prehistoric or early medieval stronghold. The site overlooked an important crossing point on the River Wye, a major artery of communications inland to Monmouth and Hereford. At the time, the Welsh kingdoms in the area were independent of the English Crown and the castle in Chepstow would also have helped suppress the Welsh from attacking Gloucestershire along the Severn shore towards Gloucester. However, recent analysis suggests that the rulers of Gwent, who had recently fought against King Harold, may initially have been on good terms with the Normans. The precipitous limestone cliffs beside the river afforded an excellent defensive location. Building work started under William FitzOsbern in 1067 or shortly afterwards. The Great Tower was probably completed by about 1090, possibly intended as a show of strength by King William in dealing with the Welsh king Rhys ap Tewdwr. It was constructed in stone from the first, marking its importance as a stronghold on the border between England and Wales. Although much of the stone seems to have been quarried locally, there is also evidence that some of the blocks were re-used from the Roman ruins at Caerwent. The castle originally had the Norman name of Striguil, derived from the Welsh word ystraigl meaning river bend. FitzOsbern also founded a priory nearby, and the associated market town and port of Chepstow developed over the next few centuries. The castle and the associated Marcher lordship were generally known as Striguil until the late 14th century, and as Chepstow thereafter. Further fortifications were added by William Marshal, Earl of Pembroke, starting in the 1190s. The wood in the doors of the gatehouse has been dated by dendrochronology to the period 1159-89. Marshal extended and modernised the castle, drawing on his knowledge of warfare gained in France and the Crusades. He built the present main gatehouse, strengthened the defences of the Middle Bailey with round towers, and, before his death in 1219, may also have rebuilt the Upper Bailey defences. Further work to expand the Great Tower was undertaken for William Marshal's sons William, Richard, Gilbert and Walter, in the period to 1245. In 1270, the castle was inherited by Roger Bigod, 5th Earl of Norfolk, who was a grandson of William Marshal's eldest daughter, Maud. He constructed a new range of buildings in the Lower Bailey, as accommodation for himself and his family. Bigod was also responsible for building Chepstow's town wall, the Port Wall, around 1274-78.