With its massive keep and walls, the castle still looks formidable, although little inside it remains. It is the oldest castle in west Wales, and for more than 300 years it was the seat of the earls of Pembroke. It was founded by the Montgomerys in 1093, and work began on the fine masonry a century later with the circular great tower, or keep. Dominating the castle, the tower stands 22m (75 ft.) high and is the finest of its type in Britain. Home to such great leaders as Earl William Marshal, regent to Henry III, and to the early Tudors, the castle was also the birthplace of Henry VII ("Harri Tewdwr"). During the civil war, the castle was held in turn for both Parliament and the king. Cromwell arrived in person to start the siege that led to its final surrender. A vast cavern underneath the castle, called the Wogan, is where food and water were stored. The water defenses of the fortress can still be traced in a millpond on the north and in marshes on the south, outside the walls, as well as in the River Pembroke over which it looms.