Concentric castles were the next follow on from stone keep castles. Concentric castles, in Britain, are most associated with Edward I and North-West Wales where a series of huge castles were built. The most famous concentric castles are at Harlech, Beaumaris, Caernarvon and Conway.
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.
Part of Caernarvon Castle complete with batter
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.
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.