The Château Fort de Sedan is the centrepiece of the town and one of the most impressive medieval fortresses in France. At 35,000 square metres spread across seven levels, it makes a credible claim to being the largest fortified medieval castle in Europe. Construction began in 1424, and the builders kept going for the better part of two centuries, adding towers, ramparts, gun platforms, and the handsome Renaissance-era Princes' Palace — which tells you something about how continuously this place was threatened.
The castle houses a well-stocked museum covering the town's military history from the medieval period through the Second World War, with particular focus on the two catastrophic Battles of Sedan in 1870 and 1940. The ramparts offer panoramic views over the Meuse and the town below, and you can walk most of the outer walls. Give yourself two to three hours to do it justice.
The castle also functions as a hotel — you can book rooms inside the old walls, which makes for an unusual night. Museum admission and the hotel operate independently; a day visit requires only the museum ticket.