Concentric Castles, the High Point in Castle Design

author icon By George Ghidrai

Last update: 26.07.2023

Concentric castles undoubtedly represent one of the high points in Medieval military architecture. In this post you will learn:

  • what is a Concentric Castle

  • why was a need to innovate castle design

  • why were these fortifications considered a high point in military architecture

  • the great advantages and drawbacks of concentric castles

  • where can we find concentric castles

3d Archaeological Reconstruction by BeanBox

What is a Concentric Castle?

The idea behind a concentric castle is a castle within a castle. Basically, the castle is surrounded by two independent rings of curtain walls: the inner wall was always higher than the outer wall, providing a vantage point for archers firing at the enemy.

From Stone Castles to Concentric Castles

Until the end of the 12th century, stone-built and earth and timber castles were widely used, but by the late 1100's, Motte-and-Bailey castles made of earth and timber began to fall out of fashion. Castle designers realized it was preferable to build in more durable stone.

However, stone castles built until the late 12th century had few defensive features: a high central keep, sometimes a water-fill moat that surrounded the castle, and some square towers along the curtain wall and flanking the gatehouse that was often poorly defended.

Stone Castle Diagram

Diagram of a stone castle built until the late 12th century

The shape of the castle would have been dictated by the surrounding land, which meant that every castle had a different design and layout.

Why the need to innovate castle design?

As castle design evolved, so did the tactics used by attackers to seize them. For example, attackers quickly discovered that they could undermine the fortifications by burrowing under the foundations; moreover, powerful siege-engines were developed (such as the catapult or the trebuchet) which were able to demolish parts of the stone walls.

the trebuchet

The Trebuchet

The small number of towers (also used as lookouts) made it difficult for defenders to determine where an enemy attack might occur, especially if the attackers were coming from various directions.

The late 12th-century innovation

It was evident that there was a need for further innovation in castle defense. From this point on, most newly constructed castles would have a regular shape: square or polygonal. Circular towers had been designed at each of the corners and sometimes these towers would have protruded outwards from the walls and featured arrow slits on each level to allow archers to target anyone nearing the curtain wall.

These new castles did not always need a keep, but where the keep did exist it was no longer square but polygonal or cylindrical. Sometimes the keep was sacrificed to save money, and in these situations, the space inside the towers would have made up for the loss of the keep and provided additional space for habitation.

Gatehouses were strongly defended and they were equipped with deadly traps to make a direct assault difficult:

  • A portcullis (which comes from the French word porte coulissante, meaning sliding door) was a wooden or (more often) metal grill that was lowered from the ceiling in front of the attackers to block the passage. A strong gatehouse could be equipped with several portcullises to block the passage and possibly trap intruders between them.

  • Murder holes were openings in the ceiling of the passage used by the defenders to pour boiling water (oil was too expensive and hard to obtain), rocks, and other heavy objects onto attackers.

  • Arrow slits were small openings in the stone wall through which archers defending the castle would fire against enemies inside the gatehouse.

All these deadly traps were used to create panic among attackers inside the gatehouse.

Concentric Castles, the high point in castle design

Concentric castles surpassed all types of stone castles in terms of their importance, defensive capabilities, and long-lasting structure. These fortifications were designed to maximize the strategic potential of every angle for medieval military purposes.

Beaumaris Castle in Wales, one of the great examples of concentric castles

Beaumaris Castle in Wales, one of the great examples of concentric castles

The great innovation was that the outer wall was not used simply to reinforce the inner one. Instead, each wrapping wall was considered individually and they were designed to be separate obstacles for invaders to overcome.

The word concentric does not imply that these castles were circular; in fact, most of them were either square or polygonal.

Because of the two separate walls, concentric castles would have had two separate wards: the outer bailey, between the inner and outer walls, and the inner bailey, inside the inner wall. The keep was not a necessary element; in fact, most concentric castles were built without a central keep.

A key point in the design was to build towers facing in all directions. In the early castles, towers were built in a square shape. However, attackers soon found out that by burrowing under one of the corners of the square foundations, they could undermine the entire tower. That is why late Medieval Castles (including Concentric Castles) had circular towers (or the so-called drum towers) with no corners which could be undermined.

concentric castle diagram

Diagram of a Concentric Castles

The great advantages of concentric castles

Concentric castles had many advantages over earlier designs. In order to take the castle, attackers would have had to penetrate at least two walls or pass through a heavily-defended gatehouse. They were often caught in the area between the two curtain walls, nicknamed 'the death area' (in fact, this is the outer bailey). Here, they were at the mercy of the archers shooting from atop the inner wall, with nowhere to hide.

Siege-engines could not be brought close to the castle, as many also had a water-filled moat surrounding the outer wall. Because of the distance to the castle (and the extra height of the inner wall), accuracy would have been quite low and the best attackers could hope for would be to breach the outer wall with their catapults.

Because of the symmetrical shape and the towers facing all directions, defenders had a much better view of the surrounding countryside.

In fact, at their peak-time, concentric castles were so formidable that attacking them directly was deemed hopeless. The only hope was to lay siege to the castle in the hope that the inhabitants would eventually starve and surrender. However, many concentric castles had their own water supply (often a well that could not be interfered with by attackers outside the walls) and could grow their own food.


Building a concentric castle was phenomenally expensive and only the King and the powerful military orders, such as the Hospitallers and the Knights Templars could afford to build or maintain such a structure. Moreover, it took such a long time to construct such a castle, that often by the time it was completed, it wasn't needed any longer.

Over time, attackers got smarter as well. Realizing that mounting a full-scale attack on such a formidable fortress was useless, they would instead try to starve out the castle by laying it to siege, making a mockery of the huge effort spent in building the elaborate fortifications.

Where can we find concentric castles?

Some outstanding examples of concentric castles:

  • Chateau de Bonaguil, a marvel of military architecture which stands today as one of the most impressive and evocative castle ruins in France.
  • The iron-ring of castles built by King Edward I in the 13th century in Wales includes three amazing castles: Beaumaris Castle, Caernarfon Castle, and Conwy Castle. Unfortunately, these castles nearly bankrupted the King.


Exploring Castles :

Castrum to Castle:


Cambridge University Press:

See also