​​In 871 King Eathelwulf and his brother Alfred fought against the Vikings at Merdon but it was not until King Stephen's brother, Henry de Blois became bishop that a castle was built on the site. In 1138, as Stephen's reign descended into civil war Henry built five castles including that at Merdon but it was never used and when Henry II came to the throne in 1153 the castle was made indefensible.

For the next four hundred years Merdon retained its role as a manorial court and a palace for the Bishop and his friends to enjoy hunting. To the south of the castle a 450 acre deer park, Hursley Park, was created surrounded by a park pale which survives to this day.