Ayr Castle, to prevent it being garrisoned by the English. Guardian William Wallace resigned as Guardian of Scotland after his defeat at the Battle of Falkirk. He was succeeded by Robert Bruce and John Comyn as joint Guardians, but they could not see past their personal differences. As a nephew and supporter of King John, and as someone with a serious claim to the Scottish throne, Comyn was Bruce's enemy. In 1299, William Lamberton, Bishop of St. Andrews, was appointed as a third, neutral Guardian to try to maintain order between Bruce and Comyn. The following year, Bruce finally resigned as joint Guardian and was replaced by Sir Gilbert de Umfraville, Earl of Angus. In May 1301, Umfraville, Comyn, and Lamberton also resigned as joint Guardians and were replaced by Sir John de Soules as sole Guardian. Soules was appointed largely because he was part of neither the Bruce nor the Comyn camps and was a patriot. He was an active Guardian and made renewed efforts to have King John returned to the Scottish throne. In July 1301 King Edward I launched his sixth campaign into Scotland. Though he captured the castles of Bothwell and Turnberry, he did little to damage the Scots' fighting ability, and in January 1302 he agreed to a nine-month truce. It was around this time that Robert the Bruce submitted to Edward, along with other nobles, even though he had been on the side of the Scots until then. There were rumours that John Balliol would return to regain the Scottish throne. Soules, who had probably been appointed by John, supported his return, as did most other nobles. But it was no more than a rumour and nothing came of it. In March 1302, Bruce sent a letter to the monks at Melrose Abbey apologising for having called tenants of the monks to service in his army when there had been no national call-up. Bruce pledged that, henceforth, he would "never again" require the monks to serve unless it was to "the common army of the whole realm", for national defence. Bruce also married his second wife that year, Elizabeth de Burgh, the daughter of Richard de Burgh, 2nd Earl of Ulster. By Elizabeth he had four children: David II, John (died in childhood), Matilda (who married Thomas Isaac and died at Aberdeen 20 July 1353), and Margaret (who married William de Moravia, 5th Earl