Dunster Castle motte, or Upper Ward, and an area below shaped to form the bailey, or Lower Ward. Somerset became more stable in the aftermath of the post-invasion period and the unsuccessful 1068 rebellion against Norman rule. It was common in the period for the Normans to build religious houses to accompany major castles, and accordingly William de Mohun endowed a Benedictine priory at Dunster in 1090, along with its parent abbey at Bath. The River Avill was important for trade; the region around Dunster was rich with fisheries and vineyards, and Dunster Castle prospered. Stone fortifications were built on the site
Dunster Castle during the early 12th century, probably forming a shell keep around the top of the motte. In the late 1130s England began to descend into a period of civil war known as the Anarchy, during which the supporters of King Stephen fought with those of the Empress Matilda for control of the kingdom. William de Mohun's eldest son, also called William, was a noted supporter of Matilda, and Dunster was considered one of her faction's strongest castles in the south-west. In 1138 forces loyal to Stephen besieged the castle; a siege castle was built nearby, but all trace of it