Guinevere was the Queen of King Arthur in the court of Camelot, and she plays a central part in the tales and folklore of the castle and the Knights of the Round Table. She was said to have fallen in love with Sir Lancelot, King Arthur's most trusted knight. This story makes up perhaps the most famous Love Triangle ever known.
Her origins are thought to be Welsh. She was said to be descended from a aristocratic family of Romans and was "the loveliest woman in all the island". In some texts, she is portrayed as Leodegrance's daughter (the previous owner of the Round Table). Her affair with Sir Lancelot is a crushing blow to Arthur. It is Lancelot, not Arthur, who rescues Guinevere from Méléagant.
Keira Knightley as Guinevere in "King Arthur"
When Guinevere and Arthur met, Merlin the Wizard comes into play. Merlin warns Arthur that Guinevere is not wholesome enough to be his wife, and predicts that she and Lancelot will have an affair, but Arthur is smitten by her, so Merlin agrees to go to Camelot to arrange the marriage with Leodegrance who delivers Guinnivere to London with the Round Table with "100 fine knights".
After the marriage, "all knights are sworn to the Round Table" and this event marks the beginning of the Court of Camelot.
Later, Guinevere is abducted by Mordred, Arthur's nephew, while he is in Europe campaigning. Sir Lancelot comes to her rescue, and he repeatedly defend her honour thoughout her life (when she is accused of having an affair with Sir Kay for example). And all the while, Lancelot and Guinevere fall hopelessly for each other, seemingly unable to stop a train of events that would ultimately lead to tragedy.
In one incident, Sir Lancelot climbs into Guinevere's bedroom by forcing the window bars and injures his hand.
In the morning Meliagrance sees a blood stain on Guinevere's sheets and claims Guinevere had been lying with one of her wounded knights. Lancelot stands up for the Queen, denying any responsibility, but protecting her honour, and Meliagrance challenges him by throwing the gauntlet, which Lancelot accepts.
Meliagrance traps Lancelot in a cave by tricking him, and then announces that Lancelot had gone off campaigning to side-step the duel. Sir Lavaine steps up to fight in his place, but Lancelot escapes, and appears to fight Meliagrance, and with one arm tied behind his back (and with no shield) he cuts Meliagrance's head in two.
Ultimately, it is Guinevere and her love of Lancelot, and their betrayal of King Arthur, that destroys the Round Table and the Court of Camelot.