Right period, wrong king:
Vortigern, usurping saxon king of england wishes to build tower. Each time he gets to the third course of bricks there is an earthquake and the tower tumbles to the ground. His soothsayers tell him to sacfrifice a child born of no man to quell the quakes. They capture the child merlin and prepare to sacrifice him, but he tells the king that below the tower is a lake, where two dragons fight. That night on merlins instructions the king pulls up the foundations, releasing the magic lake. Two dragons, white and red appear, and fight. The white is killed. Merlin then tells him that the white was him, and the red was Uther Pendragon (arthurs father). Vortigern is now able to build his tower somewhere close by but his death has been foretold, and ambrosius and his brother uther beseige him in the tower and kills him by setting it aflame.
It isnt the welsh, they hadnt been invented...the british of arthurs time WERE the welsh, you ninny. Its the Saxons they are fighting. And its a story of the young merlin, not arthur.