 Central to the imposition of Norman rule was the building of castles.There are well-preserved castle ruins at Helmsley, Pickering and Scarborough and others existed at Ayton, Danby, Mulgrave and Whorlton.In the 11th and 12th centuries monasteries were established on the moors at Whitby Abbey, Rievaulx Abbey, Byland Abbey and Mount Grace Priory.