Once William the Conqueror had firmly established his rule in England, he moved on to building more permanent castles. These were the Stone Keep or Square Keep castles. The Stone Keeps had many advantages over the Motte and Bailey castles. For example, they were made of stone so they lasted longer, would not rot and they were a lot stronger than wooden ones. Also, as a result of the stone being strong there was always the possibility to build up which meant there was the advantage of defenders seeing for miles. The walls of the Stone Keep castles were substantially thicker than those of the Motte and Bailey castles. The walls weren't just built as a defensive structure, but also for residential purposes. They did not rise in a straight line, but splayed out at the base to form a strong plinth. This added greater strength to the castle.The windows of the Stone Keeps were very high and even the lowest windows were still fairly high off the ground and extremely narrow; an archer could fire accurately from within, but attackers would rarely hit their marks from without.
Although the Stone Keeps were much larger, taller and more intimidating than the Motte and Baileys, there were disadvantages to the way they were built and the methods of attack that was used against those who defended it. There was a gradual shift towards more dynamic defensive forms in the second half of the twelfth century due to the rapid improvement in seige techniques. The revolution in castle building was largely influenced by the experiences of crusaders in the Holy Land and in Spain. Warfare became more sophisticated, enabling those attacking to beat the castles defences in a variety of ways. Scaling ladders and siege towers allowed attackers to go over the wall, catapults and battering rams could breach the wall and, worst of al, undermining could bring a section of the wall crashing down. Finally, the early thirteenth century saw improvements in the defenses of the battlements to make better use of the crossbow. The castle design was improved by increasing the number of embrasures and narrowing the merlons. At the same time, arrow slits were incorporated into the merlons so that the bowman could fire whilst remaining under cover. Significant as these adaptions were, there was one which led eventually to a fundamental rethinking in castle design and the obsolescence of the keep. This was the adoption of the flanking tower which developed from the concerns of the defense of the bailey.
This new design, called the Enclosure castles, was usually built to enclose a coutyard containing the residence of an Earl. An example of an enclosure castle is Framlingham, Suffolk. The building of the castle started in 1190 after Earl Roger Bigod obtained permission to rebuild his father's castle that was demolished by order of Henry II. Within the wall, at intervals, were built thirteen projecting towers and there was no keep at all. The importance of these towers is that they divided up the curtain wall into short secions which could be covered by crossfire from two sides and above, they also protected eachother. Another advantage was that if the enemy did reach the foot of the wall and forced the gateway, they had not captured the castle itself, because each tower was a kind of a mini keep itself. In spite of Framlingham's great strength, the castle was captured in 1215 by King John's forces.
Concentric castles are what followed on from the Enclosure castles. They are mosly associated with Edward I and North-West Wales where a series of huge castles were built. He spent a great deal of time and money to ensure he held his new lands in Wales. The most famous concentric castles are at Harlech, Beaumaris, Caernarvon and Conwy. In the process, he evolved a new style of military architecture known, not unnaturally, as "Edwardian castles". Edward's architects discarded the common keep design in favour of a concentric model which could be used as a base for offensive operations rather than as a purely defensive stronghold. Concentric castles have no central strong point like a keep. Instead they rely on rings of walls, one inside the other, with towers along the length of the walls. Most Edwardian castles have three concentric rings of walls and towers. The central space was kept as an open courtyard around which were clustered separate domestic buildings. The outer wall was ringed by a moat with access over a draw bridge through a separate gatehouse or barbican. The beauty of the Edwardian design is that the walls could be easily defended. The fortified towers built into the walls provided covering fire for each other. Defenders could provide cross-fire along the walls between neighbouring towers. Edward came up with a further innovation at Caernarfon and Conwy. Instead of a separate castle standing on a high point with the settlement spread out around it, the castle walls enclose the entire medieval town. So the castle walls are also the town walls.
The man most responsible for Edward's Welsh castles was the architect James of St. George. James was a Savoyard (from modern Italy), and he incorporated elements in his design from Europe and the Middle East. Edward consciously chose to suppress elements of Welsh tradition and heritage. Conwy Castle was built over the traditional burial place of Llewelyn the Great, and the monastery at Aberconwy was destroyed and the monks moved to a new foundation at Maenan.
Throughout this essay, I have stated the reasons that caused the designs of castles to change throughout the centuries. The reason for building the first type of castle, the Motte and Bailey, was for William the Conqueror to show off his power. The reason why those designs were later adapted was due to weakness in design against attacking forces and weather. The Stone Keep design was ruled out and the enclosed design was brought in because of, again, weakness in design but also advances in technology and warefare strategies, a change of power and new ideas from crusaders. Similar reasons are what caused Concentric castles to be built after the Enclosure castles.
I personally think that the main reason for the change in design of castles over the centuries was that there were weaknesses in the designs but all the reasons I stated are very closely linked and caused eachother. For example, change of power caused new ideas which caused development in warfare stategies and technology which then caused weakness in designs.
Nowadays, castles are enjoyed by many as they are used as tourist attractions and the process of decay for most castles have temporarily been halted.
