Built between 1445 and 1476, this was once a lighthouse but was used mainly as a jail as late as the 19th century. A low rampart connects the cylindrical tower to the Tour de la Chaîne. During the Wars of Religion, 13 priests were tossed from its summit. You climb 162 steps to the top hold; in clear weather, the panoramic view extends all the way to Ile d'Oléron. On the way up, you can still see graffiti scrawled by former prisoners.