Christchurch Castle was built in the late 11th century to protect the town 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 hall was built around 1150.
The stone keep ~ slighted after 1650 ~ replaced a wooden structure in about 1300. The castle saw action in the Anarchy around 1147 and also in the Civil War in 1645.
All that's left today are these ruins standing on the mound...