The Warwick Castle was built in 1068 by William I (William the Conqueror, or Guillaume le Bâtard), the Duke of Normandy, who conquered England and Wales. And whose impact on those two nations was so great, that its true nature is still being disputed. However, his rule brought irreversible changes to the English language, way of life and architecture. It is a fine example of Norman architecture, and although not a cheap place to visit (£18), it does offer a few interesting exhibitions, dungeons, and ramparts. There is a also a cafe and a restaurant on the grounds.
Easily reachable from Birmingham and London with Chiltern Railways - walking distance from Warwick station (a bit longer hike from Warwick Parkway station).