Vikings frequently raided Irish monasteries for their valuable treasures, killing many monks. To protect themselves, monks built tall round towers next to the monasteries to flee to when Vikings attacked. One of the most remote monasteries was on Skellig Michael island in the Atlantic, but even it was raided by Vikings in 824. Some Vikings settled in Ireland, establishing towns by the sea like Dublin, Cork, Waterford and Limerick. When two groups of Vikings wanted to claim the same beautiful land in Ulster, they raced their ships to the shore, but one man cut off his own hand and threw it ashore first to claim the land for his people.