Gloucester's strategic position on the River Severn has more than once embroiled it in England's civil conflicts. It, or at least its castle (once located on the riverbank just outside today's entrance to the Docks, but demolished 1787 to make way for a prison), was besieged multiple times in the years 1263–1265 during the Second Barons' War. In May 1322 Sir John Giffard, Second Baron of Brimpsfield, was executed by King Edward II at Gloucester (and his castle at Brimpsfield, in the Cotswolds east of the city, reduced to the small overgrown mound it is today) for his part in the seizure of the city a few months previously. In 1471, during the Wars of the Roses, Yorkist Gloucester prevented a Lancastrian army from crossing the Severn, thus setting the stage for its defeat at Tewkesbury.

On 10 August 1643, during the English Civil War fought between King and Parliament, it was King Charles the First who pitched up at the gates of Gloucester and placed the city for the last time at the centre of national affairs.

Gloucester was a solidly Parliamentarian city, the last in the Severn valley after a Royalist victory at Bristol the previous month. That victory along with success in the north had swung the tide of the civil war in King's favour in the summer of 1643, a period which would later be known as the Royalist Summer. Further victories at Gloucester and Hull would secure the west and north for the Royalists and allow the King to turn his full attention on the last Parliamentarian stronghold and ultimate seat of English authority, London.

Gloucester had other ideas. Its defiance of the King was made more emphatic by the flames that engulfed the suburbs outside the city's defences; as the Parliamentarians retreated behind medieval walls and new earthworks, they torched the suburbs to deny their enemy cover. Those suburbs remained largely undeveloped again until the boon years of early the early-19th century. A rare survivor of the destruction is the originally 15th-century lower wing of the Whitesmiths Arms opposite the entrance to the Docks on Southgate Street.

Mid 17th-century Gloucester was a relatively small city of some 5,000 inhabitants, but it had long since spilled beyond its Roman-origin medieval walls. Only the eastern half of those walls remained, the western half having been robbed of material for the construction of civic buildings inside the city and, as night fell on the first day of the siege of Gloucester, the now smouldering suburbs outside the medieval city boundary.

To south and east the city defences rested largely on the medieval walls, bulwarked to enable them to withstand the artillery of the day. The missing sections of medieval wall had been hastily resurrected in earthwork form, and an outer earthwork to the north turned the River Twyver and the Old Severn (a now extinct channel of the Severn that once ran farther east of the east channel) into a moat.

To the west, the east channel of the Severn provided a more formidable natural barrier, behind which more earthworks were erected for good measure. On the basis that lumbering siege engines and artillery generally do not float all that well, the Old Severn was dammed to flood the hams to the north-west of Gloucester.

A drawbridge had been installed on Westgate Bridge, and the other gates into the now shrunken city – the late 12th-century Alvin Gate at the top of Hare Lane to the north (believed not to have survived the siege), the outer north gate at the junction of today's Northgate Street with Black Dog and Bruton Ways, and ancient East and South Gates – were shut. A ditch dug below the eastern and southern defences completed the encirclement by water of a Parliamentarian Gloucester now encircled by Royalist foe.

Unwilling to risk the casualties incurred in the storming of Bristol the previous month, the King demurred from an immediate assault on Gloucester's walls and set about knocking them down instead. It would take, he estimated, ten days to force the city's surrender. He established camps at Llanthony Secunda Priory to the south, Barton Hill to the east and Kingsholm to the north. Except for a few brief visits to his capital at Oxford, the King conducted the siege from his headquarters at Matson House, which still stands today as a care home facility on the lower north-east slope of Robinswood Hill.

Royalist troops dug forward trenches and artillery emplacements opposite the east and south walls, on a line from the area of long-since-demolished Issold's House on Eastgate Street (near the start of the modern-day pedestrian precinct), roughly along the line of modern-day Cromwell Street and round the south-east corner of the old wall to Gaudy Green (the site of the heaviest artillery pieces, where today's still green Brunswick Square is located). This section, along today's Parliament Street and Brunswick Road up to Eastgate Street, was where the main Royalist effort would be made.

Unfortunately for the King, the biggest artillery he possessed, a mortar, proved more dangerous to his own troops than the people of Gloucester; the first time the gunners tried to fire it the treacherous thing promptly exploded and fell off the wall on which it was perched. Recent Gloucester folklore holds this event as the inspiration for the Humpty Dumpty nursery rhyme, a tradition that actually appears to be rooted in a mischievous 20th-century confection.

The rest of the Royalist artillery proved so weak that it may well have been able to float across those flooded meadows after all. The more than 400 artillery shot fired at the city during the siege did more damage to its churches than it did to the city's defences and inhabitants. Greyfriars and the already ruinous St. Oswald's Priory were particularly badly damaged, while the churches of St. Mary de Crypt (used by the Parliamentarians as a munitions store) and St. Nicholas are also known to have been struck.

Whilst the King was effectively bouncing small lumps of iron off Gloucester's walls, the city's military commander, Lieutenant-Colonel Edward Massie, was demonstrating the impetuosity of his age, all twenty-three years of it, to good effect. Undeterred by the fact that his 1,500-strong garrison was seriously outnumbered (by ten to one at the end of the siege), he repeatedly sallied out to raid the Royalist trenches, adding enemy action to their own mortar in the list of mortal dangers the King's troops faced.

As Royalist artillery ran low on ammunition and firing stones proved even less effective than cannon balls in breaking the city's walls, the King called on his engineers to undermine them. They found that the springs around Gloucester were every bit as sympathetic to the Parliamentarian cause as the city was, and progress on the main effort, a tunnel towards the east gate, was slowed by the constant need to pump out water.

Nevertheless, Royalist engineers were creeping ever closer to the walls, not only with the tunnel at the east gate but also in their efforts to fill the ditch on both south and east sides near the south-east corner of the city wall. Massie responded by erecting another earthwork defence across Friars Orchard, inside the walls at Greyfriars, ready to receive any assault there.

Things were about to come to a head, or more accurately the surface, when the Royalist tunnel reached the east gate on 4 September. After all their raiding, the garrison did not have enough ammunition to withstand the assault that would surely follow the breaching of the city's defences.

Unfortunately for the King, his much-vaunted cavalry proved they had nothing to actually vaunt about when they singularly failed to impede the progress across the Cotswolds from London of a relieving army under the Earl of Essex. The King had run out of time and was forced to withdraw from Gloucester after a twenty-six day siege he thought would end with victory in ten. While Essex's men were slipping and sliding their way down Prestbury Hill near Cheltenham, the King's men were headed up into the Cotswolds at Birdlip, the King himself having ridden on ahead to Painswick.

Although Gloucester would be a Parliamentarian thorn in the Royalist side, Royalist Worcester allowed the King to maintain lines of communication between his capital of Oxford and his Welsh base of support across the Severn. Moreover, with a large Parliamentarian army in the field, London, was vulnerable. If Essex's relief army could be defeated before it reached the safety of the Parliamentarian stronghold, the Royalist Summer may yet end in a complete Royalist victory. It was a short-lived hope. The Parliamentarian victory at the Battle of Newbury fifteen days after the siege of Gloucester ended allowed Essex and his army to return to London. The failure of the Royalist siege of Hull in October ended all hope of the King assembling enough forces to capture a now heavily defended London.

Gloucester remained a Parliamentarian stronghold almost to the end of the civil war and subsequent Cromwellian interregnum, until Massie demonstrated again his youthful impetuosity by returning to the city as an ardent Royalist in 1660 and becoming its Member of Parliament. After the restoration of the monarchy that year, King Charles II succeeded where his father had failed; he had Gloucester's remaining medieval wall torn down in retribution. He also removed much of the area surrounding the city, known as the inshire, from city control and purged any remaining Parliamentarian sentiment from Gloucester's governing corporation.

The city prudently kept its head below the parapet it no longer had and demonstrated its loyalty to the restored monarchy by erecting a statue of King Charles II in the wheat market on Southgate Street. In perhaps a more telling reveal of the city's true feelings towards the House of Stuart, the statue was mislaid when the market was demolished a century later and only rediscovered, in pieces, in 1945. It now stands, damaged and badly decayed, against a wall (naturally) in a corner of St. Mary's Square, near St. Mary de Lode, the church in which Royalist prisoners of war were detained during the siege of Gloucester.

For a while after the siege, the people of Gloucester celebrated the anniversary of the lifting of the siege, a 'Gloucester Day' tradition that has been revived in modern times as part of the annual Gloucester History Festival.