There are several pieces of evidence that suggest King Arthur could be based on a true story. The fact that there are so many different versions of the Arthurian legend across different regions of Britain indicates that there was an original kernel of truth that spread and evolved. Also, the descriptions of the society and the military aspects in the Arthurian tales bear some resemblance to what we know about the Dark Ages in Britain. Moreover, the presence of Arthur - related artifacts, though not conclusive proof, do add to the possibility that there was a real person behind the legend.
The historical context of the Dark Ages in Britain provides some evidence. There were wars and power struggles during that time, which could have given rise to a figure like Arthur. Also, the existence of early Welsh poems and stories that mention a heroic figure similar to Arthur before the full - blown legends developed is a sign that there might be a historical basis. Additionally, some ancient battle sites in Britain have been speculated to be the locations of Arthur's battles.
Some of the place - names in Britain are evidence. For example, Tintagel in Cornwall is often associated with Arthur's birthplace. Archaeological finds in that area might also be related to the Arthurian era.