King John's Castle is a castle located on King's Island in Limerick, Ireland, next to the River Shannon. The walls, towers and fortifications remain today, and are a visitor attraction. The remains of a Viking settlement were uncovered during the construction of a visitor centre at the site. The Viking sea-king, Thormodr Helgason, built the first permanent Viking stronghold on Inis Sibhtonn (King's Island) in 922. He used the base to raid the length of the River Shannon from Lough Derg to Lough Ree, pillaging ecclesiastical settlements. In 937 the Limerick Vikings clashed with those of Dublin on Lough Ree and were defeated. In 943 they were defeated again when the chief of the local Dalcassian clan joined with Ceallachán, king of Munster and the Limerick Vikings were forced to pay tribute to the clans. The power of the Vikings never recovered, and they reduced to the level of a minor clan, however often playing pivotal parts in the endless power struggles of the next few centuries. The arrival of the Anglo-Normans to the area in 1172 changed everything. Domhnall Mór Ó Briain burned the city to the ground in 1174 in a bid to keep it from the hands of the new invaders. After he died in 1194, the Anglo-Normans finally captured the area in 1195, under John, Lord of Ireland. In 1197 local legend claims Limerick was given its first charter and its first Mayor, Adam Sarvant. A castle, built on the orders of King John and bearing his name, was completed around 1200. Under the general peace imposed by the Norman rule, Limerick prospered as a port and trading centre. By this time the city was divided into an area became known as "English Town" on King's Island, while another settlement, named "Irish Town" had grown on the south bank of the river. A 1574 document prepared for the Spanish ambassador attests to its wealth: Limerick is stronger and more beautiful than all the other cities of Ireland, well walled with stout walls of hewn marble... There is no entrance except by stone bridges, one of the two of which has 14 arches, and the other 8 ... for the most part the houses are of square stone of black marble and built in the form of towers and fortresses.