Although what we think of as the Legend of King Arthur is taken from Thomas Malory (and later T.H. White) and is set in the Middle Ages, the first mentions of Arthur came centuries earlier, when he was referred to as a battlefield hero of the sixth century. The stories grew over the centuries, and many places in the British Isles have become associated with King Arthur and his contemporaries.
Tintagel is reportedly the place where Arthur was conceived (through trickery) and may also be his birthplace. The castle at Tintagel in Cornwall you can see today was built in the 1230s, but excavations reveal several earlier layers of construction. The fortress is in a popular spot for watching for invasion from abroad, and may well have been used in this manner for a thousand years or more. Tintagel Island is also the site of Merlin's Cave.
The site of the old Roman Amphitheater in Caerleon is often referred to as "King Arthur's Round Table". Legend has it that Arthur set up court in Caerleon after the Romans left Britain. Geoffrey of Monmouth noted that Arthur held court in the City of the Legions. in Welsh, Caerleon means "The Fortress of the Legion."
More possible sites for Camelot, and Arthur's resting place, after the jump.
Another candidate for Camelot is Cadbury Castle in Somerset, an Iron Age hill fort. Excavations in the 1960s find that it has been occupied and refortified several times in different time periods dating back to 500 BC, including the period of Arthur's lifetime in the sixth century.
Among other possibilities for the legendary Camelot is Caerwent, near Gwent in Wales. The old Roman fort there was certainly occupied and busy during the Dark Ages. Clues from Malory's work lead modern theorists to think this may be the real location of Arthur's court.
Bryn Myrdden, or Merlin's Hill, near Carmarthen in South Wales, is traditionally the final resting place of King Arthur's advisor Merlin. There is a cave under the hillside, where he was supposedly entombed by the Lady of the Lake after he revealed all of his magical secrets to her.
Glastonbury Tor has many legends surrounding it, including a visit from young Jesus and its location as the resting place of the Holy Grail. Some scholars identify it as the Isle of Avalon, where Arthur retreated after being mortally wounded, only to await the time when Britain would again need him.
In ancient times, the area surrounding the tor was swampland, making it a true island. When the fog rolls in, you can see how Glastonbury Tor still resembles an island.
Glastonbury Abbey is the oldest church in Britain, first built in 670-678 AD on the foundations of an older Celtic place of worship. Legend says the Christian version was founded by Joseph of Arimathea. A catastrophic fire in 1184 AD led to the lucky discovery in 1191 of a grave with an iron cross labelling the remains of King Arthur and his wife Guinevere. The remains later disappeared, and the cross is considered to be a hoax which helped finance the rebuilding of the abbey.
For additional reading, see Britannia History. Thanks to proud Welshman John Gale for additional links.