Mount Badon, if it exists at all, should appear on the map somewhere.  But where?

There are many, many possibilities.

First of all, we should note where Mount Badon is not.  For all that Geoffrey of Monmouth embellished and expanded the Arthurian legend, he did history a disservice in supposing that King Arthur ruled all of England, Scotland and Wales.  Geoffrey wrote his book under the patronage of Robert of Gloucester, who was trying to justify the rule of England by his half-sister, Maud.  Thus, because Maud had roots in Normandy, so did Arthur; because Maud was hoping to rule all of Great Britain, so did Arthur; because Maud’s power base was in and around Gloucester, so was Arthur’s.

Yet even in the twelfth century, for one king to control all of Great Britain by force of arms was extremely difficult.  Gloucester himself was unable to spread his influence east, into the territory controlled by King Stephen, whose rule centered on London.  Both pretenders to the throne had trouble controlling Ranulf, the Earl of Chester, who’d stretched his domain across the north of England from Chester to Lincoln.  Looking at the situation objectively, to think that a Dark Age king (Arthur) could have ruled all of Normandy, Wales, Scotland, and England when the Romans couldn’t do it, William the Conquerer couldn’t do it, and no King of England could do it until 1745 (with the exception of Normandy, which had been lost to France), defies all reasonable reading of history.  

Therefore, the historical Mt. Badon (if it is a true place) has to exist within striking distance of Wales–the last bastion of the Britons facing a Saxon advance–and circumscribed within an area no further east than Birmingham, south of Gloucester, or north of Chester.   

Early British Kingdoms has this to say:  http://www.earlybritishkingdoms.com/arthur/kabattles.html

“It was at the Battle of Mount Badon that tradition says the Saxon advance into Britain was finally halted. It was Arthur’s greatest victory and, not surprisingly, there are many claimants for its location. Forts are preferred since Gildas, in his De Excidio Britanniae, more properly called the battle a “siege”  . . . Possibilities include Bowden Hill, Lothian; Dumbarton Rock, Strathclyde; Mynydd Baedan, Glamorgan; Little Solway Hill, Somerset; or Brent Knoll, Somerset. Modern theory, however, suggests one of the many Badburys around the country: in Devon, Dorset, Wiltshire, Berkshire, Warwickshire, Northamptonshire and Lincolnshire. Liddington Castle, near Badbury in Wiltshire, seems most popular at present. Welsh tradition backed up by Geoffrey of Monmouth is, however, almost certainly correct in identifying the battle site with Bath, Caer Baddon, or, at least somewhere in its vicinity. Bathampton Down has been suggested.”

Bath lies just south of Gloucester.  Not a surprise, given Geoffrey’s patron.

The Welsh word for ‘Bath’ is ‘Caer Faddon’ (http://www.geiriadur.net/).  It is the Welsh translation of an English name, not the Welsh word for ‘bath’ itself, which is ymdrochfeydd.  This observation leads to another possible location for Mt. Badon, one mentioned in one the Welsh myths, The Dream of Rhonabwy.  In this poem, Arthur and his men dismount below Caer Faddon, within a half-day’s ride of the ford of Rhyd-y-Groes (ford of the cross).  The ford is not near Bath, but is an actual, well established, geographic site.  It is located on the Severn (Hafren in Welsh) River, east of Welshpool.     
http://www.donaldcorrell.com/mabinogn/rhonabwy.html

In looking at a map, or Google Earth for that matter, there are a whole host of possible locations for a Dark Age siege within a half-day’s ride of this ford, including Breiddon Hill, an enormous iron age hill fort.  The range of mountains in which Breiddon is located sits on the eastern edge of what became known as the ‘Marche’–the borderland between Wales and England–and adjacent to Offa’s Dyke, which dates to the 700s AD and was built by the Saxons to pen the Welsh into their mountains. 

Identifying the Mount Badon of the Anglo-Norman Arthurian tales with the Caer Faddon of the Dream of Rhonabwy at last connects the continental Arthur with the native Welsh tales.  It also de-links King Arthur from Geoffrey of Monmouth’s Gloucester-centric perspective and perhaps finally brings us closer to finding the geographic center of the historic, battle-leader named Arthur.