Montgomery Castle in mid-Wales had passed to the Herbert family in the early 17th century and was the home of Edward Herbert, 1st Baron Herbert of Cherbury, philosopher, diplomat, and elder brother of the metaphysical poet George Herbert. By 1644 Edward was elderly, ill, and unwilling to take part in the war that had divided his country since 1642. When Parliamentarian commanders Sir Thomas Myddelton and Colonel Thomas Mytton advanced from the recently captured Oswestry into the upper Severn valley in early September, taking Newtown by surprise along with a vital convoy of gunpowder, they came to Montgomery with little hope of an easy capture. The castle on its rock was a formidable position. But Lord Herbert simply surrendered it on terms on 5 September. The Parliamentarians moved in. Three days later, Royalists under Sir Michael Erneley and Sir William Vaughan came roaring up from Shrewsbury and caught them dispersed for forage.

Mytton pulled five hundred infantry back into the castle and prepared for a long defence. Myddelton rode out with the cavalry to bring help. The Royalists, joined by Lord Byron and possibly some of the Northern Horse under Sir Marmaduke Langdale, dug in around the castle with 2,800 infantry, 1,400 cavalry, and 300 mounted dragoons. They built earthworks and trenches and settled into a formal siege. Meanwhile Myddelton's appeals brought reinforcements from every direction. Sir William Brereton gathered Cheshire infantry. Sir John Meldrum diverted forces from his siege of Liverpool. Sir William Fairfax, cousin of the great Yorkshire commander Sir Thomas Fairfax, brought a contingent down from the army in Yorkshire. By 17 September a relieving force of two thousand infantry and fifteen hundred cavalry had assembled north of Montgomery under Meldrum's overall command, with Offa's Dyke covering one flank and the River Camlad guarding the other.

Byron formed up on a hill northwest of the castle, leaving a detachment to guard his siege works. There was no action on 17 September. Both armies sat in the cold autumn air looking at each other. On the morning of 18 September, when about a third of the Parliamentarian cavalry rode off for forage, Byron ordered an attack to seize Salt Bridge over the Camlad and cut off the relieving force's line of retreat. Colonel Marcus Trevor's cavalry drove back the Parliamentarian horse. The Cheshire infantry under Brereton rallied and held the line. Then the Parliamentarian foragers returned, the cavalry remounted, and Myddelton's horse counter-charged and broke Trevor's regiment. Brereton's foot drove back the Royalist infantry. Behind Byron's army, Mytton's garrison sallied out of the castle and overwhelmed the troops left to guard the siege works. The Royalist line collapsed inward from both sides. Five hundred of them were killed and fifteen hundred taken prisoner. The English Civil War in North and Mid Wales effectively ended that afternoon.

Many of Byron's defeated men were already survivors of Marston Moor in July. Six of his regiments had come back from Ireland after the king's negotiated armistice with the Confederate Catholics and had been chewed up twice in three months. The remnants of five infantry regiments numbered barely five hundred men by the time they fought at Naseby the following June. Byron held Chester for another year before that city too fell. The Royalists kept other castles in North Wales for a while, including Myddelton's own seat at Chirk, but they could never again raise a field army in the region. Lord Herbert went back to his books. Lord Byron's name passed down to his great-great-grand-nephew the poet, who would never visit the field where his uncle's army died. The castle that the battle was fought to save was slighted by Parliament in 1649 to prevent it being used in any future war. Fairfax's body was carried home to Yorkshire. His widow received Parliament's thanks. The hill where he led that last charge is still farmland, looking north across the Camlad toward Offa's Dyke and the English border he died defending the cause of.