The Knight of the Cart - A Merlin Mystery, #5

The embittered Sir Meliagaunt is overlooked by King Arthur when a group of new knights, including Gildas of Cornwall, are appointed to the Round Table. In an ill-conceived attempt to catch Arthur's notice, Meliagaunt kidnaps Queen Guinevere and much of her household from a spring picnic and carries them off to his fortified castle of Gorre, hoping to force one of Arthur's greatest knights to fight him in order to rescue the queen.
Sir Lancelot follows the kidnappers, and when his horse is shot from under him, he risks his reputation when he pursues them in a cart used for transporting prisoners. But after Meliagaunt accuses the queen of adultery and demands a trial by combat to prove his charge, Lancelot, too, disappears, and Merlin and the newly-knighted Sir Gildas are called into action to find Lancelot and bring him back to Camelot in time to save the queen from the stake.
Now Gildas finds himself locked in a life-and-death battle to save Lancelot and the young girl Guinevere has chosen for his bride.