"Tintagil castle by the Cornish sea..."
--Tennyson, Idylls of the King

There are differing legends of the birth. One goes: Initially held by Gorlois, it was besieged by an obsessive Uther Pendragon, in an effort to gain Gorlois' wife, Igraine. The story usually has Gorlois on the battle field while Merlin magicks Uther into the castle, where Uther sleeps with Igraine. The spell on Uther convinces Igraine that she is receiving her husband into her bed. She conceives Arthur this night. Uther returns to the battle and slays Gorlois the next day. Igraine marries Uther soon after-- but not soon enough. This brings the question of Arthur's legitimacy into question, and Merlin spirits the baby away soon after its birth to protect it.

Another version of the legend, from Tennyson's Idylls: The night Uther died, "moaning and wailing for an heir," Merlin and Bleys went down to the ocean and witnessed the wreck of "a ship, the shape therof/ A dragon wing'd, and all from stem to stern/ bright with a shining people on the decks..." As the ship went down, a series of huge waves brought a baby to shore that the two old sorcerors decided would be the next King.

Why all the mystery around Arthur's birth? Simply, it's best if your hero has a mysterious arrival on Earth-- it's better to have a miraculous supernatural being in charge than just an ordinary guy, isn't it? We can attribute much to Geoffrey of Monmouth's sense of drama. Another possibility is that the real Arthur, whoever he was, was not either legitimate or noble, and his biographers have tried to make their hero look better by making his origins mysterious but nevertheless royal.