Geoffrey of Monmouth (c1095-c1155) may bear the name of a Welsh town, but his best-known work, the History of the Kings of Britain was composed at Oxford Castle, where he appears to have been a secular canon at St George's College. This largely fictitious "history" contains, among other things, the myth of the founding of Britain by Brutus of Troy. who supposedly landed at Totnes and went on to found the towns of Winchester and Shaftesbury.
Winchester was also mentioned in The Prophecies of Merlin, an earlier work whose text was incorporated into the History. It was said to be the site of a spring that would break forth into three rivulets which would divide the island of Britain into three parts. The well-known prophecy of the battle between red and white dragons is probably the origin of the red dragon on the Welsh flag and the white dragon flag used by some English nationalist groups.
And on the subject of dragons, book 8 of the History tells how Uther Pendragon, after a victory in battle, ordered two gold dragons to be made, one of which he kept, and the other placed in Winchester Cathedral. This story may be related to the wyvern flag of Wessex in some way.
Geoffrey helped codify much of the Matter of Britain. He may have been worthless as a historian, but as a mythmaker, he was arguably on a par with Homer.