The 12th-century Welsh monk Geoffrey of Monmouth recounts a famous legend in his Historia Regum Britanniae in which the child prophet Merlin witnesses the Romano-Celtic warlord Vortigern attempting to build a tower on Mount Snowdon to keep safe from the Anglo-Saxons, but the tower keeps being swallowed into the ground. Merlin informs Vortigern that underneath the foundation he has built is a pool with two dragons sleeping in it. Vortigern orders the pool to be drained, exposing a red dragon and a white dragon, who immediately begin fighting. Merlin delivers a prophecy that the white dragon will triumph over the red, symbolizing England's conquest of Wales, but declares that the red dragon will eventually return and defeat the white one. This story remained popular throughout the 15th century.