Chepstow Castle is set high upon cliffs above the River Wye, where it guarded the main river crossing from Southern England into Wales. It was one of the first stone castles in the country, built within a few years of the Battle of Hastings (1066), for William fitz Osbern, who had been made Earl of Hereford by William the Conqueror.

The castle was further developed during the first half of the 13th century by the Marshall family, and then later that century by Roger Bigod III, with more modifications made during the Tudor period. During the Civil War (1642-48) it was twice besieged and fell to Parliamentary cannon. After the war, the defences of the castle were reinforced and remodelled for the use of cannon and muskets. The castle was used as military barracks and as a detention centre for political prisoners, most notably housing Henry Marten who spent 20 years in captivity in the tower that now bears his name. The castle finally fell into disuse after 1690 when the remaining troops from the garrison were withdrawn.