Within the castle walls, the Musée des Beaux-Arts holds a collection that traces European painting from the 15th to the 20th century. The strongest holdings are Dutch and Flemish — there are excellent examples from Rubens and his circle — but the collection also covers Italian Renaissance painting, French work from Poussin through the Impressionists, and a good suite of 20th-century pieces.
The building itself, a 1971 structure designed to fit inside the medieval castle enclosure, is more functional than beautiful. But the collection makes it worthwhile, and the location within the castle grounds means you can combine it with a walk on the ramparts without backtracking. Free on the first Sunday of each month.