As Halsall's pieces on the end of the Roman Empire in the est seem to have gone down well I thought I'd just highlight his pieces on "The Age of Arthur". Again these date back some way and were originally published in Wargames Illustrated. They also clearly form the basis of much of his "Worlds of Arthur" book - https://www.amazon.co.uk/Worlds-Arthur-Facts-Fictions-Dark-ebook/dp/B00B9D2NS4/
This is the first part - https://darkagewargaming.wordpress.com/2011/01/16/the-age-of-arthur-post-roman-britain-400-600-for-the-historical-wargamer/
You can navigate to the subsequent parts from links at the bottom of each page.
Parts 1 to 3 are a detailed review of the evidence for the period - those less interested in this may wish to jump to part 4 which starts to look at the evidence for the military.