Sir Lancelot of the Lake was one of the most trusted knights of King Arthur 's Round Table , but this soon changed when he fell in love with the king 's wife , Queen Guinevere . Sir Lancelot ( or Launcelot ) du Lac ( , , , or ; and or ) is one of the Knights of the Round Table in the Arthurian legend . He is typically considered to be one of the greatest and most trusted of King Arthur 's knights and plays a part in many of Arthur 's victories .