At the opening of the Civil War knowledgeable commanders on both sides were well aware that very few of the towns, houses and castles of England were tenable in modern war. Many sieges on the Continent – in Italy, the Dutch Wars, and then the Thirty Years War – had demonstrated that medieval walls were no match for artillery. Indeed as early as 1589 Paul Ive in his Practise of Fortification had offered the opinion that, ‘Townes enclosed with weake walles of stone, and defended with small, square or rounde towres, are insufficient to abide the mallice and offence that an enemey at this day may put in practise, the cannon being an engine of much more force than any before it invented.’ It would not be exaggerating to say that the cannon had already revolutionised siege warfare, and that in general English towns and castles were poorly prepared for what was coming. Only prodigious quantities of earth, in the latest bastioned traces, stood any chance in a protracted siege involving trains of battering ordnance. Those few places that could boast even relatively new artillery defences usually faced outward, against foreign enemies. These exceptions to the general rule of total unpreparedness were the great ports and the border extremities: Berwick, Hull, Portsmouth, some of the Cinque Ports, and certain seaside castles of the south and the Channel Islands.

William Georges, reviewing England's defences in the reign of James I, came to the gloomy conclusion that even here the forts were not of the latest design, nor were the personnel who manned them likely to make serious resistance even if properly equipped: