Unlike square keep castles, concentric castles had no central keep. In many senses, they had no centre either as all parts of the castle would have been considered to be a strong point. Each concentric castle had a very heavily defended entrance and the central core was defended by a series of curtain walls. The furthest of the curtain walls would have been the smallest in height to allow the defenders to see an approaching enemy. The curtain wall nearest to the castle would have been the highest to give the defenders the maximum height advantage over those attempting to take over the castle.
Towers in castles such as Beaumaris and Caernarvon were not square such as those found in stone keep castles. The towers at Beaumaris are circular while Caernarvon Castle has a variety of shapes (primarily polygonal) - but none of Caernarvon's are square. Round towers were difficult for the enemy to dig under with the prospect of collapsing them (called sapping) and engineers/architects from the time found that a circular shape gave towers far more strength than the traditional square ones. At Caernarvon, towers had towers built within them making them a formidable defensive feature.
However, concentric castles had two major weaknesses. They were massively expensive to build and if an attacking army decided to ignore them, troops within concentric castles had the choice of either staying where they were and not involving themselves in combat or leaving their place of safety and fighting on open ground. That stated, concentric castles were built in highly strategic areas and an invading army would usually have had no choice but to attack. All of Edward's castles were built by the sea which allowed boats as large as 300 tons to get right up to the castles to enable them to be supplied. This meant that the traditional way of defeating a castle - by besieging it - was no longer a viable option for attackers. To supply Rhuddlan Castle, Edward ordered that engineers divert the River Clywd. Records show that 968 diggers/ditchers straightened the river so that his boats could sail up the Clywd to supply the castle. This engineering feat would be a formidable task now, but Edward had it done in just three summers.
One of the finest example of a concentric castle is at Caernarvon in Wales. Here the walls are, in fact, two walls with the hollow middle between them filled in with rubble. The blocks of stone had to be extra large and strong to cope with the huge pressure put on them when the rubble was put in. This, among many other issues, gives some idea as to the importance Edward I put on Caernarvon. When this castle was finished it had cost Edward £27,000 (thought to be about £35 million or more at today's prices). This was roughly his income for one year - invested into just one castle. The wars against the Welsh had already cost Edward £100,000 and to help pay for all of this he raised taxes. Edward kept very detailed records that show how much the castles cost. Caernarvon, Harlech and Conway cost between them £50,000 - this was at a time when a skilled worker earned between 3p and 4p a day. Caernarvon and the other north Welsh castles were designed by Master James of St George, an architect from Savoy. It is probable that Edward met Master James as he returned from the Crusades as we know that he stopped off in Savoy and he was also related to the family that ruled Savoy.
Castles as homes
Edward died in 1307 and Master James in 1309. With these two deaths - one a king who wanted strong well-built fortifications, the other a superb architect - castle building in England and Wales faltered. By 1327, castle building in Wales ended forever. They had simply cost too much and kings after Edward were more interested in palaces as opposed to castles.
Large stone castles were built in Europe from about the 1100's to about the 1500's. These huge buildings served not only to defend the country from foreign invaders but as the basic tool in preserving the king's and the nobles' power over the land. The social system was very rigid in the Middle Ages.
Under Feudalism, the basic social structure in this time, all land was held by the king. The king gave pieces of this land to various high nobles, in return for their help in fighting his wars or in putting down rebellions. Not only did the higher nobles have to fight for the king themselves, they had to supply a certain number of lesser lords and other knights to help fight also. These higher nobles then gave some of their land to lesser knights, in return for their help in battle. Below all the knights were the serfs, who actually farmed the land. They gave a portion of their crops each year to the lord who ruled over them, in return for use of the land and protection.
The king could not be everywhere in the country, especially with the poor roads and the limited transportation of the Middle Ages. The king's vassals, the lords, however, could be all over the country, with their castles as symbols of their power for all to see.
A man's son inherited his lands and his obligations to fight. As time went on, inheritances became complicated, because there were lords who had no living children, who had only daughters as heiresses, and who split their inheritances among their sons (rarely daughters). When the daughter of a lord married the son of another lord, the young couple inherited land from both families. If the overlord from whom they got one piece goes to war with the overlord from whom they got the other piece-on which side did they fight? If there are two possible heirs to the throne itself, for whom do they fight? If a higher lord rebels against the king, does his vassal fight for the lord, or for the king? Who is closer, and more likely to take away his castle and his land? What will the other knights do? Which families is he allied to, by marriage or other bonds?
The castle was both a residence for the lord and his family, and a fortification. It was a strong place for the lord to defend himself against his enemies (and the king's enemies, and his overlord's enemies), a safe place for him and his knights to return to, and a place to live which emphasized his power. A few heavily armed knights could control a large area, if there was no organized army to go against them. Not only did knights fight against foreign enemies, they fought a lot against each other, and they put down rebellions among the peasants. Showing that you had a lot of power sometimes made actual fighting unnecessary. In Britain, many of the castles are along borders, to stop raids by the Welsh and the Scots, and as a basis for raiding in return.
Stone and wood were about the only building materials available. Slate and thatch (bundles of reeds or other plants in a thick bundle) were used for roofs, but not for walls. Fortunately, northern Europe had large amounts of both wood and stone. Wood didn't last as long, but, worse, it could be set on fire by the other side. Stone is very strong in compression (stone can hold up a great deal of weight). Mortar and gravity kept the stones in place. Once a stone building is constructed, it needs very little maintenance and lasts a long time. It is not, however, very pleasant to live in-a stone castle is cold, damp and dark. Many pieces were added to improve the castle as a residence.
Castles were built to keep out enemies. When an attack was expected, the drawbridge was raised, the gates and portcullis were closed, and archers were stationed on the towers. The walls were not only high, in a well-planned castle, but they were arranged as much as possible so that anyone climbing the walls could be shot at from two directions. Many castles have strange shapes because the castle was designed to accommodate the terrain, and to catch attackers in a crossfire.
The castle's defenses invited a great deal of ingenuity from the attackers. Rolling wooden towers, covered with thick hides to stop arrows and kept wet so they could not be set on fire, were brought up to the walls in an attack. Sometimes they even worked. Catapults threw heavy stones at the walls to make a breach or loads of rocks (or diseased livestock, or fire bombs) over the walls. The battering ram-generally used against a door-was an old favorite.
Thoughts of these different 'siege engines' were always on the minds of the castles' designers. The castle was often built on a raised platform. Roads to the castle angled and sloped to restrict the easy use of battering rams and the like. There was often also the traditional moat (left behind from digging out the earth to make the raised platform for the castle) and drawbridge, just to keep things interesting.
Another method of defeating a castle was laying siege to it, by trying to starve out the inhabitants, or waiting until they ran out of water. If their water could be poisoned, they had to surrender. A good well was extremely important to a castle.