This illumination from a manuscript of the French Verse Chronicle of 'Roman de Brut' by Wace, Canon of Bayeux, now in the British Museum, shows Vortigern finding the red and white dragons locked in battle.

Though the dragons are more like birds here, with their feathered wings and long talons, the crumbling tower is very nicely depicted. You can actually see the walls coming down, with the trancepts slipping.

Vortigern, dismayed, can only look on in horror.

Click the image to enlarge.