The Vikings initially ventured into the Irish Sea as raiders and took, from the monasteries and other rich centres, wealth in the form of goods and slaves. In the course of the tenth century, however, they became permanently established and founded and developed the first towns in Ireland, often under sufferance from the local population. From these towns they controlled the trade routes along the western coasts of Europe through the Irish Sea — routes that brought luxury goods to both the North and the South. The increasing economic power of the Irish towns was one of the factors that led to the English conquest of Ireland in 1170.