If you would like to see the chimney, See my other posting. Christchurch Castle was built in the late 11th Century to protect the town's and river access to the interior of Wessex. It was built inside the Saxon fort whose walls were probably used to build the castle mound or motte.
The luxurious Constable's House has the oldest Norman chimney in Britain and was built around 1150. The stone keep (slighted after 1650), replaced a wooden structure about 1300. The castle saw action in the Anarchy about 1147 and in the Civil War in 1645.