On 21st February 1645 the Shropshire Parliamentarian committee drew together forces from the garrisons of Wem, Moreton Corbet and Stoke preparatory to an attack on the Royalist garrison at Shrewsbury. This followed the weakening of the garrison's forces by Prince Maurice's withdrawal of men to assist in the relief of the Parliamentary siege of Chester. As Maurice advanced up the Welsh side of the river Dee, Sir William Brereton, commander of the Parliament forces besieging Chester, despatched a force of 350-400 horse, 250-300 foot, and a company of his own men, under the command of Colonel Bowyer, from the north side of the river to meet Colonel Mytton, who commanded the Shropshire horse, near Wem. Brereton's forces, along with the five hundred men under Mytton, numbered a total of around twelve hundred men who were also equipped with petards and other implements useful for the storming of the town. This force, the foot under the command of Lieutenant Colonel Reinking, proceeded to march towards Shrewsbury, with Colonel Bowyer in the rear.

Shrewsbury, besides being described as 'the strongest garrison in Prince Maurice's Association' (J Phillips) and one of the strongest in the country, was vitally important to the Royalist cause due to its proximity to North Wales, with its overwhelming support for the King. Therefore, to control the county was imperative as a means of keeping open links with this important recruiting area and also to the north west counties, including Chester, with its all important contact with Ireland and Charles' forces there. An earlier attempt upon Shrewsbury had been made by troops from the Wem garrison on the night of 14th-15th February, following information from a Parliamentary spy by the name of Hewson, described as a 'Minister, of Irish descent', regarding Maurice's weakening of the garrison. This attempt had failed to reach the town until daybreak due to inclement weather, and the difficulties of marching in the dark had led to them getting lost.

The force now gathered, still acting on Hewson's information, marched through the night of 21st February, arriving at Shrewsbury between three and four in the morning. Shrewsbury, standing in a horseshoe bend of the River Severn, with a strong castle, was described at the time as 'having a strong wall and a great ditch on the north-western side'. This ditch was defended on the town side by a further earth breastwork and a stout wooden palisade. At about five in the morning a boat, possibly specially made for the purpose and under the command of Colonel Reinking, was launched with up to thirty men in it, at least eight of them carpenters. They landed under the breast works, near to the palisades. When challenged by the sentries, they claimed to be friends. This seems to have led to some confusion and a delay of around fifteen minutes before the sentries fired upon them. During this time the carpenters had dismantled the palisades, allowing the attacking force to climb the fortifications, kill the sentries and enter the town.

The first troops to enter were forty dismounted troopers, led by Hewson, Captain Williers and Lieutenant Benbow, armed with pistols and firelocks. They were supported by a party of musketeers, who went along the Severn and breached the town wall near to the Council House (Sir William Owen's house). Meanwhile, Lieutenant-Colonel Reinking had entered the town at the head of three hundred and fifty men. He split his forces, some going straight into the market place, where they met some resistance from the court of guard there, before overcoming them. The rest of the force marched down to Castle Foregate, where they subdued the defenders and let in the remainder of the army under Colonels Mytton and Bowyer, along with the Shropshire Committee. The troops were then despatched along various streets with the view of finding and separating from the royalist forces in the castle, the officers and men of the garrison. During this time they took prisoner the governor and other town dignitaries, many of whom were still in their beds, along with some booty. They were accompanied in this endeavour by members of the committee, who were anxious that the townspeople, many of whom had been sympathetic to their cause, would not be plundered or harmed by the soldiers.

Further evidence of their determination to prevent plunder and unnecessary bloodshed had been the prior awarding of £2,000 from the Committee, through Lieutenant-Colonel Reinking, to Colonel Bowyer's Cheshire and Staffordshire forces. The same amount was awarded to the Shropshire forces for the taking of the town, with extra reward according to valour, but also a strong emphasis that anyone found plundering would, besides losing his share of the reward, also be subject to court-martial for his life. Despite this, Malbon claims that the castle was robbed of a quantity of plate deposited there.

The castle and a small outwork at Frankwell held out until about noon, when Frankwell surrendered with bare quarter, and the castle under terms. These terms included the English troops to be allowed to march to Ludlow with their arms, but the Irish to be surrendered to the Parliamentary forces to be summarily hanged. They were also forced to leave the ammunition in the castle and the soldiers in the town, along with their officers. Besides the large number of Royalist gentry and senior officers taken during this action, who were eventually sent to London, the booty included: fifteen artillery pieces, two thousand arms, a hundred barrels of gunpowder, money and plate worth £40,000, 'Prince Maurice's Magazine and Carriages and Baggages of the Prince'.

This victory, with the loss of only two soldiers from the attacking force and six defenders was remarkable, given the formidable nature of the fortifications at Shrewsbury, and gives some circumstantial evidence to Cave's theory that the townspeople betrayed the garrison to the Parliamentary forces. This betrayal theory is backed by Symonds, who on listing the 'Garrison of Shropshire', describes Shrewsbury as "Betrayed to the rebells in winter, 1644". Also by Malbon, who describes the disaffection between the townsfolk and a "drunken…discontented, half mutinous soldiery". Cave and Malbon appear to view this betrayal, along with the methods used by the Parliamentary troops as an ungentlemanly trick, beneath soldiers of true valour. Whether this is, on Cave's part, in order to deflect blame from his Shropshire Royalist contemporaries towards the treachery and underhand tactics is open to debate. However, as noted earlier, there does appear strong evidence to support the treachery theory. It also seems that, whilst the Parliamentary sources used may be biased, and paint a 'holier than thou' picture of the behaviour of the Parliamentary soldiers, they do all appear to tally in their description of event.