The Battle of Roslin (February 24, 1303) was one of the earliest skirmishes in the First War of Scottish Independence. The battle involved, according to some sources (incl. the Clan Sinclair website), approximately 8,000 Scots, and up to 30,000 highly trained English troops. The trigger for this altercation seems to be jealousy, on the part of Sir John Segrave, who served as Edward I of England's commander in Scotland. The object of Segrave's desire was Lady Margaret Ramsay of Dalhousie, who had fallen in love with Sir Henry Sinclair of Rosslyn.
Segrave, based in Carlyle England, learned of Margaret's intention to marry Henry, and quickly obtained permission from Edward to invade Scotland. A Cistercian prior known as Abernethy is said to have learned of Segrave's movements, and dispatched monks to warn various Scottish nobles. Several significant Scottish leaders answered the call, including John Comyn, William Wallace, Henry Sinclair, and Simon Fraser, who was elected to lead the Scottish forces. The battle ended with an absolute rout of the English, and Henry happily married Margaret.
Roslin Castle figures prominently in Walter Scott's "The Lay of the Last Minstrel":
"...With war and wonder all on flame,
To Roslin's bowers young Harold came,
Where, by sweet glen and greenwood tree,
He learn'd a milder minstrelsy;
Yet something of the Northern spell
Mix'd with the softer numbers well..."