Penhow Castle, situated in the countryside between Chepstow and Newport, claims to be the oldest inhabited castle in Wales. It was built as a home for one of the knights who served the Lord at Chepstow Castle. Sir Roger de St Maur was the first Norman owner of Penhow and he built a tower house in which to live. Documentary evidence shows that he was at Penhow by 1129. It was the first British home of the family who would later rise to national prominence under the more familiar name of Seymour.

A curtain wall enclosed a small courtyard beside the stone tower. Over the centuries more comfortable dwellings were built beside the tower, leaving very little space in the courtyard and incorporating or replacing most of the curtain wall so that only the length on the east side remains. The range to the south is a medieval manor house that received alterations up to the Tudor period. In the late 17th century the north range was replaced by a new house, built for the Lewis family.

By 1714 the castle was owned by absentee landlords, and it was tenanted as a farm. The castle was largely derelict when it was bought in 1973, but the new resident owner made extensive restorations and opened the castle to the public. In 2003 the castle was sold and is now a private residence not open to the public.