The history of Limerick stretches back to its establishment by Vikings as a walled city on King's Island (an island in the River Shannon) in 812, and to the granting of Limerick's city charter in 1197. King John ordered the building (1200) of a great castle. The city was besieged three times in the 17th century, culminating in the famous 1691 Treaty of Limerick and the flight of the defeated Catholic leaders abroad. Much of the city was built during the following Georgian prosperity, which ended abruptly with the Act of Union in 1800. Today the city has a growing multicultural population.