The Château de Caen is one of the largest medieval castle/fortress in Europe and its mighty walls, overlooking the city, have protected castle of William the Conqueror since 1060. Taking a walk around the ramparts, visiting the 12th-century St-Georges chapel and the Échiquier (Exchequer), dating from about the year 1100 and one of the oldest civic buildings in Normandy, is pretty amazing.
The largest portion of the ruins can be visited free of charge, great if you are a budget traveler. On the inside of the walls there is a Modern Arts Museum but due to traveling with small kids we could not visit it. The sculptures outside of it however are very interesting which led me to believe that the inside is too. The museum is also featuring a rooftop cafe that offers nice views of the castle walls.
Walking outside of the castle was enough to stir our kids imagination and ask allot of questions so I really recommend traveling with kids here. Twelve century fortresses are never boring and it will be interesting for everybody, young and not so young.