Sedan sits on the Meuse river in the Ardennes, close to the Belgian border — best known for the Château Fort de Sedan, which claims to be the largest medieval fortified castle in Europe. The castle is genuinely impressive: 35,000 square metres spread across seven levels, from medieval dungeons up to the Princes' Palace, with ramparts offering sweeping views over the town and the Meuse below. It doubles as a hotel, if you want to sleep inside the walls.
The town carries heavy historical weight. The Battle of Sedan in 1870 ended the Franco-Prussian War with the capture of Napoleon III and 100,000 French soldiers — France's largest military capitulation and the event that toppled the Second Empire. In 1940, German forces crossed the Meuse at Sedan again, breaching France's defences in the move that would bring down the country in six weeks. These two catastrophes shadow the town's otherwise peaceful streets.
Walk down to the botanical garden along the old bastion for a quieter hour, and make the short drive out to Bazeilles to see the House of the Last Cartridge — a moving memorial to the desperate rearguard of 1870 that most visitors miss entirely. The nearby town of Charleville-Mézières, about 20 km south, is worth the short detour for its arcaded Place Ducale.