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.
For additional reading, see Britannia History. Thanks to proud Welshman John Gale for additional links.