It doesn’t really matter what Mount Badon really was for the purposes of this discussion. Rather, it matters what it was.
Mount Badon was a tall hill, a mountain if you will. From the top of the hill, you could survey the countryside, see where your enemies were and where your reinforcements were. You could see ranges of cattle and sheep were. You could see which direction the wind was blowing by observing smoke from nearby camps.
You could also dig in and fight yourself a right good fight with infinitely less effort than those who were exhausting themselves charging up the hill to get at you. This is the twin benefit of high ground: You get to choose the battlefield. You can make it as difficult as you want for your enemy to get to you. You can build earthworks or stone walls. You can set traps for unwary scouts. You can set ambushes from behind rocks and trees. You can even charge downhill if you want, taking the initiative and the element of surprise away from your attackers. Taking and holding the high ground in battle is the ultimate defensive strategy.
There is some debate among historians over who held the high ground on the day that Arthur and the Britons won the Battle of Badon Hill. If it was indeed the Britons who had control of the hill and held off wave after wave of Saxon advances, then the achievement is remarkable and illustrative. However, the achievement was even more remarkable if the Saxons themselves were in control of the hill.
Then there is the siege element. One source calls it the siege of Badon. If it was indeed a siege, then one side would have encamped at the base of the hill and starved the other side out of food and supplies. It could very well have happened that the hungry, weary holders of the high ground came down off their mountaintop perches and barreled right into the besieging army simply because the hilltop supplies had run out.
As with many of these elements of this part of ancient British history, we just don’t know for sure. Historians, in an effort to prove their cases, form into camps sometimes as numerous as the Iron Age tribes that populated Britain before the Romans came. Whatever the case, whoever controlled the hilltop, it was the high ground that was of supreme importance that day, for whoever held it at the end was in control of both armies and the countryside for as far as the eye could see.