A concentric castle was one of the most famous forms of a medieval castle. It was the kind of castle that had two rings of outer walls with the inner wall usually higher than the outer one. The inner wall was made higher so that it could be defended effectively from the outer wall.
Contrary to what the term “concentric” might imply, the shape of concentric medieval castles was not circular but rectangular or sometimes irregular polygon. Another feature distinguishing the concentric castles is that they generally did not have a central free-standing keep.
The history of concentric castles can be traced back to ancient times when Assyrians, Persians, and Egyptians built them. In Europe, concentric castles became particularly popular during the high medieval period after the Crusades when rapid advancements were made in siege technology.
Concentric medieval castles were also popular in medieval England during the reign of Edward I when huge castles were built particularly in North-West Wales.
A variety of materials was used in the construction of a concentric castle. The most important was of course stone which could be mined in quarries. Stone in several of these medieval castles was also used from the ruins of old Roman fortifications.
Other materials that were commonly used in the construction of concentric castles included hard chalk, flintstone, sandstone, and limestone. Lime mortar was used to keep the stones in place and consisted of a mixture of water, coal, and lime.
Since a concentric castle consisted of outer and inner walls, this kind of medieval castle provided solid defense. The inner wall could be defended from the outer wall and once the outer wall was penetrated by the enemy, the inner wall could still be used to protect the actual castle.
The rounded or polygonal-shaped keeps were particularly important because they eliminated the risk of mining underneath the corners which was a huge advantage. Another important factor for defence was the presence of multiple walls, with the innermost wall being the highest.
This allowed the defense of the medieval castle to be established on multiple levels at the same time, making it virtually impenetrable.
Medieval concentric castle walls were one feature that distinguished them from other forms of medieval castles. The outer wall was called the “curtain wall” and contained projecting towers that served as the first line of defense in case of an attack.
The inner walls of the medieval concentric castle were commonly higher which allowed defenders of the castle to observe the activity on the outer wall and plan their defensive strategy accordingly.
A concentric castle was generally considered the most secure form of the medieval castle due to its solid defense. However, concentric castles had their weaknesses as well. To begin with, massive resources were needed to build these castles.
They also required more workers and more time to be built and it was common for concentric castles to be built in several years. With larger garrisons and more soldiers, prolonged sieges could sometimes result in starvation.