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.
The walls of the castle were severely damaged in the Siege of Limerick 1642, the first of five sieges of the city in the 17th century. In 1642, the castle was occupied by Protestants fleeing the Irish Rebellion of 1641 and was besieged by an Irish Confederate force under Garret Barry. Barry had no siege artillery so he undermined the walls of King John's Castle by digging away their foundations. Those inside surrendered just before Barry collapsed the walls. However, such was the damage done to the wall's foundations that a section of them had to be pulled down afterward.