Clinging to a limestone outcrop, Château de Joux, 10km north of Métabief, used to guard the route between Switzerland and France. Its foundations date to 1034, though medieval towers and Vauban-era walls have fortified it over subsequent centuries.
Audioguides and printed information are available in multiple languages. Guided tours, full of gripping tales, are in French but groups of 10 or more can book a foreign-language tour (reserve well in advance).
There's also an impressive 700-item collection of 18th- and 19th-century weapons (undergoing renovation when we passed through).
Less than 1km south of the castle along the main road is the Auberge du Château de Joux, a worthy stop for its modern, monochrome guest rooms and restaurant, open daily, whose great-value menus (from €14) grant access to a magnificent dessert buffet.