The death knell for many castles in England was the Civil War(s) of the 1640s. The Civil War arrived at an awkward point for the the traditional medieval castle: modern gunpowder weaponry had rendered their defences almost entirely obsolete as fighting positions for the major armies, yet their walls and garrisons remained frustrating obstacles for local forces skirmishing across the countryside on both sides. For some castles, such as Farleigh Castle near Bath, simply the threat of a siege by a superior force was enough to make the castle surrender, and indeed Farleigh changed hands twice during the war without shots actually being fired, but in many cases, the resistance of a castle garrison meant that an army would be forced to either commit to a lengthy siege or divert its artillery train in order to deal with it.

Some castles were actively destroyed as a result of the fighting: Wardour Castle was heavily damaged by Parliamentarian bombardment during its first siege in 1643 and surrendered after just 5 days. A Royalist counter-attack in November of the same year lacked artillery but attempted to mine under the surviving walls, accidentally destroying most of what remained and forcing the entire castle to be abandoned in May of 1644. Many other castles were deliberately 'slighted' following their capture, usually by Parliamentarian forces who wanted to prevent their falling back into Royalist hands but didn't want to be forced to establish disparate garrison forces. The Royalist Corfe Castle in Dorset, for example, was stormed following a Parliamentarian subterfuge in March 1645, after which the town council of Poole (which had declared for Parliament) petitioned the Parliamentarian forces to demolish the castle to ensure their safety, and to seize anything of value that they could in order to maintain Poole's garrison and defences. A similar situation occured in Bristol. Bristol's medieval castle had been one of the largest and most powerful in England. During the Royalist siege of the city in 1643, the castle itself successfully repulsed infantry attacks the South of the city, but was insufficient to defend the rest of the city from Royalist forces to the North and surrendered. By Fairfax's Parliamentarian siege of the city in 1645, Prince Rupert's forces were simply insufficient to defend the city's outlying forts from encirclement, and the Royalist forces surrendered without the castle even seeing combat a second time. Given that the castle had proved an obstacle in 1643, Parliament ordered the castle demolished following the end of the war.

So why weren't castles rebuilt and reinhabited? Usually because it was just far more convenient to do something else. Bear in mind that, by the 1640s, castles weren't just militarily obsolete, but their heavy defensive stonework was also outmoded, cold and cramped compared to the new domestic construction standards of the day. Sites like Haddon Hall near Bakewell show how, even before the Civil War, noble houses were already moving away from the heavy stonework of castles towards the light, airy galleries and open rooms of stately homes. For those families which lost their castles during the Civil War, it was often a blessing in disguise. While the interregnum was a difficult interlude, the return to favour and wealth for many families which accompanied the Restoration of 1660 enabled those families to build new and modern houses from scratch, rather than face the expensive and daunting task of adapting their old, and frequently heavily damaged, castles into modern homes. The Bankes family who owned Corfe Castle, for example, built a new house at their restored estate at Kingston Lacy rather than attempt to restore Corfe Castle. The new location allowed for a large and spacious mansion, which still forms the core of the current house behind its regency facade, surrounded by extensive parks and grounds, compared to the small chambers and tight confines of Corfe Castle itself. Similary, the Arundell family who owned Wardour Castle had gained sufficient finance by the 18th Century to 'rebuild', building a modern, palladian mansion adjacent to the original castle. In Bristol, the previous site of the castle was rapidly coopted into increased urban space for the growing port city, with Castle Street being a major commercial and domestic street until the destruction of the city centre in the Blitz of December 1940.