By a parliamentary ordinance, dated 11 June 1643, Myddelton, who had by that time returned to London, was appointed Sergeant-Major-General for North Wales. On 10 August he reached Nantwich in Cheshire, where he was joined by Sir William Brereton, 1st Baronet. They proceeded on 4 September to Market Drayton, and on 11 September to Wem, which they seized, garrisoned, and made their Shropshire headquarters. While they were still engaged in fortifying Wem, Arthur Capell, 1st Baron Capell of Hadham, with reinforcements from Staffordshire, marched on Nantwich, but was defeated outside Wem in two separate conflicts, on 17 and 18 October. After this victory Brereton and Myddelton left Nantwich on 7 November, were joined at Stretton by Sir George Booth with troops from Lancashire, and crossing the River Dee at Holt, entered North Wales, where Wrexham, Hawarden, Flint, Mostyn, Mold, and Holywell were taken. But all these place were given up quickly after the landing at Mostyn on 18 November of about 2,500 royalist soldiers from Ireland, and the leaders were criticised. Myddelton's troops were militiamen, while his opponents were trained soldiers.