Dinas Emrys is a rocky, steep-sided natural hill rising to over 250' on the south side of the Nantgwynant valley in Llyn Dinas, Snowdon, between Beddgelert and Capel Curig. The hill dominates the surrounding terrain. Giraldus Cambrensis speaks of it, saying, "At the head of the Snowdon Mountains, not far from the source of the Conway, which flows from this region towards the north, stands Dinas Emrys; that is, the promontory of Ambrosius, where Merlin, sitting on a rock, prophesied to Vortigern."
   Lady Guest tells us the tale of Vertigern's tower at Dinas Emrys as follows:-
   "Vortigern being forced to retire from his kingdom, in consequence of his various delinquencies, took refuge in Snowdon; and finding Dinas Emrys an eligible spot, commenced building a tower there. But, to his great dismay, he found that whatever he built in the daytime, always fell down in the succeeding night. Having consulted his magicians upon the cause of this mystery, they told him that unless he could find a child without a father, and sprinkle the tower with his blood, it would never stand. Upon this Vortigern despatched messengers in every direction to search for the required victim, and at length they lit upon Merlin, whom they brought to Vortigern, that he might be slain. But the boy exposed the ignorance and imposture of the magicians, and caused the ground to be dug at the foundation of the building, where they found two sleeping dragons, one white and the other red. These dragons awaking from their sleep commenced a furious conflict. The white one at first had the advantage of the red, but at last the red dragon prevailed, and expelled his opponent. Merlin then informed them that the red was the British dragon, and the white one that of the invading Saxons.
   "Then it was, according to Geoffrey and the Brut, that Merlin uttered the celebrated prophecy concerning the fate of Britain. Vortigern departing thence to seek some other place of refuge, bestowed that citadel upon the wonderful child, who declared his name to be Merlin Ambrosius, and after whom the spot was called Dinas Emrys."
   Lludd and Llevelys in the Mabinogion states that when Lludd ruled Britain, three plagues attacked the land. The second was a scream heard each May day that destroyed the will of the people and killed the young and laid the land barren. Lludd went to his beloved brother Llevelys who gave him the information to fight the plagues. The second plague was caused by the battling dragons of the Merlin prophecy above that represented the British and the Saxons. Lludd captured the dragons per his brothers instructions and buried them in a cistern at his strongest fortress Dinas Emrys.

   There is evidence of human occupation dating from the late Iron Age. A triple stone rampart can be barely discerned today from the air. The first recorded exploration of the site was recorded by Major C E Breeze in 1910. He worked at the base of the square tower and amongst his finds were 12 gold-plated bronze studs and a gold-plated bronze bar. The footings of a rectangular stone tower 36' x 24' stand on a rock above a circular platform from the 9th century or earlier, linked with the pool and medieval cistern. In the area of the pool, Breeze found a bronze coated iron terret buckle and a piece of a second of the type common to the 1st century CE. Stretches of rampart can still be seen at the site as well as the base of the square tower, thought to be 12th century. Later excavations in 1954-56 by the archeologist Dr H. N. Savory suggest that the inner rampart forms an almost separate summit fort and probably date to post-Roman times of our legend. The platform above the pool was as described in the Historia Britonum but date much later than the accepted time of the legends so we can not say whether the existence of the cistern platform inspired the story or whether it was built in response to the story. Savory described the fortifications as consisting of stone walls between 8-10' thick enclosing an irregular area of about a 10,000 m² in size. The original fortress access was by a steep path on the western side with the present northeastern entrance added much later.