In the 9th century, the city was repeatedly attacked by the Vikings, who sailed up the Seine on great fleets of Viking ships. They demanded a ransom and ravaged the fields. In 857, Björn Ironside almost destroyed the city. In 885–886, they laid a one-year siege to Paris and tried again in 887 and in 889, but were unable to conquer the city, as it was protected by the Seine and the walls of the Île de la Cité. The two bridges, vital to the city, were additionally protected by two massive stone fortresses, the Grand Châtelet on the Right Bank and the "Petit Châtelet" on the Left Bank, built on the initiative of Joscelin, the bishop of Paris. The Grand Châtelet gave its name to the modern Place du Châtelet on the same site.