In February 1644 Myddelton's command in North Wales was confirmed by a fresh commission. He left London about the end of May 1644, and marched to Nantwich, and thence to Knutsford, where a muster of all the Cheshire forces was intended, against Prince Rupert and Lancashire. But about 4,000 royalists laid siege to Oswestry. Myddelton hurried there before the arrival of his colleagues, and raised the siege on 2 July. Returning to Nantwich, Myddelton for some time watched Prince Rupert's movements, making occasional raids into Montgomeryshire. On 4 September he captured the garrison at Newtown, and the same day advanced to Montgomery, and without any resistance Montgomery Castle was surrendered to him by its owner, Edward Herbert, 1st Baron Herbert of Cherbury. Sir Michael Ernely, who was in command of the royalist forces at Shrewsbury, marched upon Montgomery to recover it. Myddelton sallied out to collect provisions but Ernely intercepted his return, and defeated him outside the town. Myddelton's foot-soldiers, under Colonel Thomas Mytton, succeeded in re-entering the castle, which Ernely at once besieged; but Myddelton retired to Oswestry, and after obtaining reinforcements from Lancashire returned, accompanied by Brereton and Sir William Fairfax. They arrived on 17 September in sight of Montgomery, where the whole strength of both parties in North Wales and the borders was now assembled. After a desperate conflict, in which Fairfax was mortally wounded, the parliamentarians completely routed their opponents.