The Battle of Naseby, in June 1645, had broken the Royalist field army. By the end of July, Rupert - whose reputation as the war's most brilliant cavalry commander had once been unassailable - was writing to Charles urging him to seek peace by treaty. The king refused. He still had men, he still had munitions, and he believed Bristol could hold. The plan was to use the port to receive fresh recruits raised in Wales. On 3 September, Rupert convened a council of war and laid out three options: break out with the cavalry, abandoning the infantry; retreat to the inner walls, abandoning the city's population; or defend the outer defences. The first two were unacceptable. They chose the third, knowing the outer walls were too long for the men he had to hold them.

On 22 August, the first of Fairfax's units arrived under harassment from Royalist patrols, one of which captured Colonel John Okey. The main body came up on the 23rd and dug in at Stapleton. Fairfax did something unusual for an army of the period: he insisted his troops pay in cash for everything they took from local people. The effect was immediate. Local 'clubmen' - country folk who had armed themselves to protect their villages from both armies - began to drift into the Parliamentarian camp. Around two thousand of them joined Fairfax. On the 28th, the clubmen took the fortress at Portishead from its Royalist garrison, closing off any hope of relief from the sea. The next day, Fairfax threw a bridge over the Avon and encircled the city completely. Nights brought sham attacks designed to wear down gunpowder and morale.

On 31 August, Fairfax intercepted a letter from Lord Goring saying he could not arrive to relieve the siege for at least three weeks. On 4 September, Fairfax sent Rupert a formal summons to surrender, with a personal note appealing to factions within the king's court that opposed Rupert - the party led by George Digby. Historians disagree about how much Rupert was moved by these arguments. What is clear is that he stalled: first asking for the king's permission to surrender, then for Parliament's sanction of the summons, then opening negotiations he did not seem to be conducting in good faith. By 10 September, Fairfax had lost patience.

At two o'clock in the morning on 10 September, four great guns opened fire and the assault began. The outer walls fell quickly, as everyone had known they would. Priors Hill fort held for three hours - the defenders there fought from a strongpoint with scaling ladders coming up at them on all sides. When it fell, no quarter was given and the whole garrison was killed. In the south, the clubmen pushed the Royalists back toward the city centre. The defenders retreated to the inner walls and set fires in preparation for surrender. Before eight o'clock - less than six hours after the first shot - the trumpet sounded for ceasefire. The lives lost at Priors Hill were neighbours, soldiers, men with names recorded in regimental rolls and in the memories of families across England, on both sides of a war that had become a national catastrophe.

Charles was stunned. He had told himself Bristol would hold for months. Now it had held for hours. He dismissed Rupert from all his offices and ordered him to leave England. 'You assured me you would keep Bristol for four months,' the king wrote bitterly to his nephew. 'Did you keep it four days?' Some historians have argued Charles was so committed to the impossibility of losing Bristol that he could only explain its fall as treachery. Digby fed that suspicion at court, suggesting Rupert had been bribed. The truth was simpler. Bristol's walls were too long and too thinly defended; Fairfax's army was the most formidable England had ever raised; the war was over and the king had not yet accepted it.

The fall of Bristol left Chester as the only major port still connecting English Royalists to Ireland. Resistance in the south collapsed. The king clung to a few fortified towns for nine more months before surrendering to the Scots. When news of the surrender reached London, the Commons voted to restore Nathaniel Fiennes to his seat - he had been disgraced for failing to hold Bristol in 1643, but it was now clear the city had been less defensible than anyone had assumed. Oliver Cromwell, writing to the Commons as he had after Naseby, argued that his men had fought and died for religious liberty and deserved a voice in the settlement. The factional bitterness those letters helped stoke would end, three years later, in the military coup known as Pride's Purge.