The Tudors concentrated their efforts on maintaining the defences of those parts of the kingdom which were vulnerable to an external threat: the south and east coasts, and the Scottish border. Berwick's Elizabethan defences were built in the new style, with a complex system of bastions designed to allow enfilading fire to sweep the approaches to the ramparts. At Carlisle, however. Henry VIII adapted the medieval castle to mount artillery by lowering and broadening the walls. He was also responsible for the construction of a series of fortifications along the south coast (for example, at Deal, Portland and Walmer) which, though not castles in the strictest sense, are generally referred to as such: though designed to mount powerful batteries of guns and more compact than earlier defences, they still presented almost as sizable a target for artillery as some of their medieval predecessors. At Dartmouth and Kingswear, Henry again adapted fifteenth century castles to fit into his defensive scheme.

When civil war broke out in place of the foreign invasion which had long been seen as the principal threat to England's security, neglected and dilapidated medieval fortifications regained much of their former importance. They had been built to dominate strategic points and to act as bases for garrisons who would exercise control over the surrounding country, posing a constant potential threat to any opposing field army which might enter the area and hampering its operations by compelling it to devote a significant part of its strength to the neutralisation of fortresses. These functions were no less valid in 1642 than they had been four hundred years earlier and, as the French army would discover at Verdun in 1916, even a neglected fortification is infinitely preferable to no fortification at all.

Few castles had ever seriously been considered to be impregnable; even if they were more vulnerable to seventeenth century artillery than to the siege equipment of the Middle Ages their effectiveness had simply been reduced - it had by no means disappeared. Indeed, it might even, in a sense, have been increased: a trebuchet or siege tower might be assembled on the spot by carpenters, but not a cannon, which would have to be transported to the scene of operations at the cost of infinite labour over roads no better than those which had existed when the castle was first built, together with its powder and shot. A contemporary author [4] advised that "if the way be foule, morish and durty, then for a whole cannon weighing 7000 lb weight, you must spanne in 15 couple of horse and a Thiller." A "quarter cannon" or culverin, probably the smallest weapon which was of any real use in battering walls, [5] required 6 pairs of horses [6] plus the single "thiller"; it weighed about 3000 lb. Even in good weather, it would have been no sinecure navigating the gun and its cumbersome team along the unmade roads of the period, together with carts containing all the paraphernalia needed to keep horses, gunners and gun mobile and functional: progress would hardly have been rapid at the best of times. Ram; narrow bridges; hills and movement across rough terrain would have reduced it to a crawl [7] if possible, therefore, heavy artillery would be moved by sea or simply borrowed from the fleet, the Parliamentarians obviously having the advantage here due to their superiority at sea.

For example, in June 1648, Hugh Peters obtained two drakes two culvenns and two demi-culvenns from the "Lion" at Milford Haven for use in the siege of Pembroke, [8] while Monck deployed ten naval guns to supplement his small battering train at Dundee in 1651. [9]