Until 1651, the Principality of Sedan belonged to the La Tour d'Auvergne family. It was at that time a sovereign principality. Their most illustrious representative, Marshal Turenne, was born at Sedan on September 11, 1611. With help from the Holy Roman Empire, it managed to defeat France at the Battle of La Marfée, though immediately afterwards it was besieged and its prince, Frédéric Maurice de La Tour d'Auvergne, duc de Bouillon, submitted to France. Only a year after that submission, it was annexed to France in return for sparing his life after he became involved in a conspiracy against France.
Due to this major victory, which also made the "Second Reich" of Germany possible, September 2 was declared "Sedan Day" (Sedantag) and a national German holiday in 1871. It remained a holiday until 1919.
Today Sedan is known for its castle, that is claimed to be the largest fortified medieval castle in Europe with a total surface of 30.000 m2 on 7 levels. Construction started in 1424 and the castle's defences were constantly improved over the ages. It is the only remaining part of the once enormous fortifications in and around the town.