The Chain Tower

The Chain Tower (1384) (French:La tour de la Chaîne) along with the Lantern tower and the Saint Nicolas Tower, is one of the three medieval towers guarding the port in La Rochelle, France. It is called the Chain tower because an actual chain was stretched across the port entrance from this building. In 1879 the French government classified it as a Monument historique (MH). 
History.
This tower along with the Saint Nicolas Tower stood at the entryway to the Port of La Rochelle. The way this tower got its name: at times throughout history a chain was stretched between the two buildings to stop ships from entering. Throughout history the building was also used to store gunpowder. The tower has not changed much since the 14th century.