He is best known as the author of a polemical work (c.550) De excidio et conquestu Britanniae (“The Ruin and Conquest of Britain”) in which he attacked the British for their wickedness. In spite of its rhetorical tone and historical inaccuracies it is the only substantial written source for the condition of Britain during a crucial period. It contains an account of a British victory over the Saxons at Mount Badon, possibly a site in Dorset, around 500. Nennius later claimed that King Arthur had fought at the battle.