The 15th-century lighthouse-cum-prison on the seaward city wall at La Rochelle, France. It has several other local names, of which the most common is 'la tour des Quatre Sergents' based on the fact that two (of four) army sergeants involved in a plot against the restored French monarchy is the 'La Rochelle conspiracy' of 1822 were imprisoned there before their trial and execution in Paris that year: all four are regarded as democratic republican martyrs. It is the only surviving medieval lighthouse on the Atlantic coast and has been classed as a French historic monument since 1879.