Concentric Castles, the High Point in Castle Design

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 there a need to innovate castle design

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

• the significant 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.

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) that could demolish parts of the stone walls.

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 importance, defensive capabilities, and long-lasting structure. These fortifications were designed to maximize the strategic potential of every angle for medieval military purposes.

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 designed to be a separate obstacle 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 design point was building towers facing in all directions. In the early castles, towers were built in a square shape. However, attackers soon discovered that burrowing under one of the corners of the square foundations 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 that could be undermined.

The great advantages of concentric castles

Concentric castles had many advantages over earlier designs. Attackers would have had to penetrate at least two walls or pass through a heavily defended gatehouse to take the castle. 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 relatively 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 plan was to lay siege to the castle, hoping 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 food.

Drawbacks

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

Over time, attackers also got smarter. 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 a 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 is a marvel of military architecture; today, it is one of France's most impressive and evocative castle ruins.
• The iron ring of castles built by King Edward I in the 13th century in Wales includes three impressive castles: Beaumaris Castle, Caernarfon Castle, and Conwy Castle. Unfortunately, these castles nearly bankrupted the King.

See also

• Decline of the Castles and the end of the "Castle-Story"

• How did Castles appear?