The castle is high on a cliff above the River Wye, as it guards one of the main river crossings from England into Wales. The strategic position of Chepstow had been appreciated by the Romans – the arch above the main doorway is made from Roman brick brought from a fort that once stood nearby. Chepstow’s strategic position allowed defenders to supply the castle via the river during a siege. Historically it was probably the first stone castle to be built anywhere in Britain. There were a number of alterations to the castle over the ages – from the later Normans to the Tudors and through to the Cromwellians.
William fitz Osbern was created earl of Hereford by William the Conqueror, and was given the Welsh Marches to control. By the time he died in 1071 he had built the rectangular keep, which still forms the core of the castle today. At the end of the 12th century, Chepstow passed by marriage to William Marshall, Earl of Pembroke. He set about bringing improving the fortifications, rebuilding the east curtain wall which was the vulnerable side of the castle, with two round towers projecting outwards The arrow-slits in the towers, designed to give cross fire on the ground in front of the curtain, were one of the earliest examples of the then new defensive idea
Because Chepstow was built in stages along the river Wye, the castle not built in the usual concentric layout, instead being constructed in a long, terraced pattern.
By 1245, the Earl of Pembroke’s sons had both enlarged Chepstow’s defences and improved the internal accommodation. They added a new lower bailey, a twin-towered gatehouse, a strongly defended barbican, plus making. additions to the Great Tower.