The Royalist artillery proved inadequate for the task of siege work and, faced with a shortage of ammunition, the besiegers attempted to breach the city walls by mining.

With Royalist miners about to reach the city's east gate and the defenders critically low on gunpowder, a Parliamentarian army led by the Earl of Essex arrived and forced Charles to lift the siege.

Following the Royalist defeat at the Battle of Edgehill in October 1642, King Charles I's control of the south was limited to Cornwall, Wales and the Marches, and a pocket of the Thames Valley around Oxford, where he based his wartime capital.

[2] The ruling Puritan elite of Gloucester ensured the city of some 5,000 inhabitants would be from the outset a staunchly Parliamentarian stronghold in the struggle against an anti-Puritan King and his Catholic Queen Henrietta Maria.

[4] An unsuccessful attempt to rally Royalist support in Cirencester in August 1642 prompted Parliamentarians in Gloucestershire to muster the militia, and in November the first regular troops were garrisoned in Gloucester.

[5] In February 1643 the Royalist commander Prince Rupert easily defeated a militia garrison at Cirencester, prompting the Parliamentarians to recall their outposts at Berkeley, Sudeley and Tewkesbury.

Later that month an army of 2,000 Welshman led by Lord Herbert of Raglan arrived on the west bank of the Severn opposite the city, but lacked the strength for an assault across the river.

The strategic situation since Edgehill had improved significantly for the Royalists, with victories in the north of England, the arrival of reinforcements and materiel in June, and an opposition that had fallen into disarray.

[9] Rupert's plan to consolidate the Royalist grip on the west by seizing Gloucester faced opposition from those in the King's camp, led by his wife Queen Henrietta Maria, who believed a march on London could end the war.

Gloucestershire – populous, wealthy and Parliamentarian – could be punitively taxed, and Welsh manpower and money could be freely deployed against the King's enemies in the rest of the country.

This compromise plan pleased neither the Queen's faction, who saw it as an unnecessary diversion from the more decisive objective of London, nor Rupert, who remained in Bristol in the belief that the Royalist army was not strong enough to allow Massey to honourably surrender without a fight.

Most decisively, the Earl of Newcastle, whose support was necessary in any campaign against London, declared himself unable to march south while Hull remained in Parliamentarian hands, leaving Charles too weak to contemplate such a move.

Lord Forth, who had been given overall command of the siege, set up a fortified leaguer to the rear of the ruined Llanthony Secunda Priory and established his forces to the south of the city.

[30] By this time, the reinforcements from Herefordshire and Worcester under the command of Sir William Vavasour, 1st Baronet of Copmanthorpe arrived on the west bank of the Severn and began crossing the river to take up positions at Kingsholm, to the north of the city.

[34] The Royalist bombardment continued through 14 August, but a breach blown in the wall by the larger guns on Gaudy Green was quickly plugged by the defenders with woolsacks and gabions.

Miners recruited from the Forest of Dean were set to work under a protective gallery to fill the 12-foot (3.7 m) deep by 30-foot (9.1 m) wide moat which, when completed, would allow them to undermine the walls.

Constrained by the limited supplies of ammunition and gunpowder, the bombardment had little material effect, though it prompted Massey to hastily erect a breastwork in the open ground of Friar's Orchard, at the south east corner of the city where the Royalist artillery was being concentrated.

An attempt that day by Massey to disrupt it with a sally from the north gate was beaten back by cavalry, but as before, a shortage of ammunition – at one stage the Royalists resorted to firing stones – blunted the impact, and little damage was done to the city.

[42] Following news that the Earl of Essex was raising a Parliamentarian army in London, Charles and Rupert travelled to Oxford on 26 August, where they held a two-day council of war.

Believing that Essex could muster no more than 6,000 troops, the decision was made to continue the siege, though contingency plans for the blockade of Gloucester were drawn up in case it had to be lifted.

His route was dictated by the desire to minimise the amount of open country favourable to cavalry operations he would have to pass through, and took him north of Oxford via Aylesbury and Bicester and then across the Cotswolds.

[47] Massey continued to harass the besiegers with sniping and artillery fire, and he resumed countermining operations at the east gate after a reconnaissance party had reported Royalists miners were still working on their mine.

On 1 September he fortified Friars Orchard with an earthwork wall and sconce, a fort in which he placed four pieces of artillery that could be brought to bear on any Royalists who broke through the south-east corner or the east gate.

The Parliamentarian army was largely untroubled by Wilmot's Royalist cavalry, and because of the differing rates of march it was strung out as far back as Chipping Norton, nearly 6 miles (9.7 km) to the east.

Meanwhile, Rupert had led the rest of the Royalist cavalry from Gloucester and, having rendezvoused with Wilmot, ended the day with some 5,000 men camped around Bourton-on-the-Water, 4 miles (6.4 km) south-west of Stow.

Massey's aggressive tactics were a critical factor in the successful defence of the city, and ensured that morale never dropped to the low levels witnessed before the siege began.

In a feint designed to draw Charles away from his intended route back to London, Essex marched north to Tewkesbury, where he could better supply his army and threaten Royalist Worcester.

[57] Essex believed he had established a commanding lead over Charles and, not wishing to drive his hungry army too hard, slowed his pace on the march to Swindon.

[58] He was further slowed after Swindon when Rupert's cavalry caught up, and the resulting Battle of Aldbourne Chase on 18 September allowed the rest of the Royalist army to close the distance.

[60] Gloucester resumed its role as a logistic base for local operations and remained a Parliamentarian centre for the rest of the war, though Massey was removed as governor in 1645 over suspicions about his loyalty.