The castle that William the Conqueror built around 1060 still dominates the centre of Caen, its massive walls enclosing an area the size of several city blocks. Entry to the grounds is free, and you can walk the ramparts for views over the city and climb what remains of the 12th-century keep. Inside the walls sit two good museums — the Musée des Beaux-Arts and the Musée de Normandie — plus a restaurant and pleasant gardens.
The scale of the place is impressive: the enclosure covers about 5 hectares, and the curtain walls running 600 metres around the perimeter are mostly intact despite the 1944 bombardment. The castle grounds are used for outdoor events in summer. A walk here before visiting the city's churches and museums sets the medieval context well.