Caerwent as a trading centre, and been used by both Saxons and the Welsh. The Lancaut and Beachley peninsulas, opposite Chepstow, were in Welsh rather than Mercian control at that time, although by the time of the Domesday Book Striguil was assessed as part of Gloucestershire. The Normans After the Norman conquest of England, Chepstow was a key location. It was at the lowest bridging point of the River Wye, provided a base from which to advance Norman control into south Wales, and controlled river access to Hereford and the Marches. Chepstow Castle was founded by William fitzOsbern, 1st Earl of Hereford, in 1067, and its Great Tower, often cited as the oldest surviving stone fortification in Britain, dates from that time or shortly afterwards. Its site, with sheer cliffs on one side and a natural valley on the other, afforded an excellent defensive location. A Benedictine priory, now St Mary's Church, was also established nearby. This was the centre of a small religious community, the remains of which are buried under the adjoining car park. Monks, originally from Cormeilles Abbey in Normandy, were there until the Dissolution of the Monasteries. The castle was expanded by William Marshal in the late twelfth century and, a century later, by Roger Bigod, 5th Earl of Norfolk. Bigod was also responsible for establishing a weekly market and annual fair, in the town which had grown up on the slopes between the castle and priory, and for building the Port Wall around it shortly after 1274. A toll gate controlled entry to the market area; this Town Gate was rebuilt in the 16th century. The town faced some hostile attacks from the Welsh to the west, but after the 14th century the castle's importance diminished. The port of Chepstow developed during the mediaeval period, one reason being that its control by a Marcher Lord, rather than by the King, meant that it was exempt from English taxation. It mainly traded in timber and bark from the Wye Valley, and with Bristol. From mediaeval times, Chepstow was the largest port in Wales; its ships sailed as far as Iceland and Turkey, as well as to France and Portugal, and the town was known for its imports of wine. Later history Chepstow was given its first charter in 1524, and became part of Monmouthshire when the county was formed. The town appears as "Strigulia", "Chepstowe" and