How reliable is Geoffrey of Monmouth's account of King Arthur? Why is he considered the founder of the legend?

<Tab/>For thousands of years the tales of King Arthur's exploits have enthralled people from all over the world. Arthur is undoubtedly one of the greatest legendary figures ever created and he has provided England with a heroic icon; a symbol of a glorious past rival to that of say Greece or Rome. <Tab/>He has been reborn to the world century after century from the pen …

…Geoffrey's claims are obviously fictitious, including the majority of his 'evidence' concerning Arthur, the fact remains that he didn't invent Arthur. The impact of Geoffrey's work on the medieval world and subsequent Arthurian romances means that his name will be found in every book written about the Arthurian legend. His work is the logical starting point for any historian looking for the truth about Arthur, which is an admirable achievement for the "father of lies".