It was now felt that some more serious measures must be taken with this terrible enemy. Argyll's pursuit of Macdonald had slackened since he learned that the Irish had joined hands with Montrose; but he was still laboriously plodding after them with three thousand of his Campbells. To these were now added two regiments from the regular army in England, and a strong force of cavalry. These reinforcements did not quicken Argyll's pace, but they prevented Montrose from giving battle to a foe whom above all others he longed to meet on a fair field. His little force had been still further diminished by the despatch of Macdonald with half his men into the Western Highlands to beat up recruits among the men of his name, who had been from time immemorial the sworn foes of Mac Callum More. For some weeks therefore the nimble Cavalier was forced to content himself with leading his heavy-footed, heavy-hearted pursuer a chase backwards and forwards through the Grampians, till at last he rested at Fyvie Castle on the Ythan, in the north-east corner of Aberdeenshire.
Here he was nearly caught napping. Argyll had crept within two miles of the castle before Montrose knew that he had so much as crossed the Grampians. It was a dangerous moment for the King's Lieutenant. He dared not risk a battle on the open ground: the castle would not stand a siege; and to make matters worse, the Royalists had run short of ammunition. But behind the castle rose a rugged hill, crowned with trees and roughly defended with some farm enclosures. Here Montrose stood at bay. The pewter vessels of the castle were melted into bullets, and the first attack of the Covenanters supplied the defenders with powder. Argyll's men were half-way up the hill, when Montrose, turning to a young Irishman whom Macdonald had left in charge as his lieutenant, said, "Come, O'Cahan, what are you about? Take some of your handiest fellows, drive those rascals from our defences, and see that we are not troubled with them again." The brave young soldier rushed down the hill at the head of his men, drove the Covenanters pell-mell before him, and returned in safety with several bags of powder. As they came back into their lines, one of the Irishmen said, looking at the powder, "We must at them again; the stingy rogues have left us no bullets." The jest was soon made earnest. As Argyll's foot retired, the horse came up on another side against the little body of Royalist troopers. But Montrose saw the movement, and leading some of his musketeers round the brow of the hill, poured so hot a volley into the Covenanting squadrons, that they descended the slope much faster than they had climbed it. This was enough for Argyll. He withdrew his men beyond the Ythan, and Montrose marched back unmolested to Blair-Athole.