Mount Badon is the supposed site of one of Arthur's twelve battles against the Saxons. Nennius recounted the twelve battles in the ninth century. The following passage is part of Nennius's description of the twelve battles:
...but the (Saxons) flying away unto the wood of Caledon, being besseged by the Brittaines, were contrayned to yeeld themsleves: and pledges being taken for tribute yearely to be paied, he gave them leave with their shippes onely to returne into their countrie. Afterwardes within a short time the Saxones were ashamed of the league made: and having recovered their strength, they made their league as voyde, and beseeged the the citie Badon rounde aboute, which is now called Bathe: this when Arthure hearde of, havinge gathered his hoaste together, and beholding the tentes of his enemies, he spake thus unto them.
"Because the most ungodly Saxones, disdaine to keep promise with me, I keeping faith with my God, will endeavore to be advenged of them for the bloud and slaughter of my citizens: Let us therefore manfully set upon those traytours whom by the mediation of Christ out of all doubt we shall overcome with a wished triumphe." And hee rushing upon the ranckes of the Saxones, beinge helped by the prayers of Dubritius in overthrowing many thousandes, obtayned the victorie: and the few which fled this garboyle, he caused them to yeelde to his mercy.....
---excerpt from Nennius's Historia Brittonum, translated by Richard Robinson
The photo depicts a hill-fort known as Badbury Rings, in Dorset, a massive Iron Age hill-fort built near an important Roman road. This fort may have been what was then known as Badonic Mount.
An aerial view of the hill-fort known as Badbury Rings (Thomas, 1971)