A concentric castle is a castle

Castle

A castle is a type of fortified structure built in Europe and the Middle East during the Middle Ages by European nobility. Scholars debate the scope of the word castle, but usually consider it to be the private fortified residence of a lord or noble...

 with two or more concentric curtain walls

Curtain wall (fortification)

A curtain wall is a defensive wall between two bastions of a castle or fortress.In earlier designs of castle the curtain walls were often built to a considerable height and were fronted by a ditch or moat to make assault difficult....

, such that the outer wall is lower than the inner and can be defended from it. The word concentric

Concentric

Concentric objects share the same center, axis or origin with one inside the other. Circles, tubes, cylindrical shafts, disks, and spheres may be concentric to one another...

does not imply that these castles were circular; in fact if taken too literally the term "concentric" is quite misleading. The layout was rectangular (at Belvoir and Beaumaris) where the terrain permitted, or an irregular polygon (at Krak and Margat

Margat

Margat, also known as Marqab from the Arabic Qalaat al-Marqab is a castle near Baniyas, Syria, which was a Crusader fortress and one of the major strongholds of the Knights Hospitaller...

) where curtain walls of a spur castle

Spur castle

A spur castle is a type of medieval fortification that uses its location as a defensive feature. The name refers to the location on a spur projecting from a hill...

 followed the contours of a hill.

Concentric castles resemble one castle nested inside the other, thus creating an inner and outer ward

Ward (fortification)

In fortifications, a bailey or ward refers to a courtyard enclosed by a curtain wall. In particular, an early type of European castle was known as a Motte-and-bailey. Castles can have more than one ward. Their layout depends both on the local topography and the level of fortification technology...

. They are typically built without a central free-standing keep

Keep

A keep is a type of fortified tower built within castles during the Middle Ages by European nobility. Scholars have debated the scope of the word keep, but usually consider it to refer to large towers in castles that were fortified residences, used as a refuge of last resort should the rest of the...

. Where the castle includes a particularly strong tower (donjon), such as at Krak or Margat, it projects from the inner enceinte

Enceinte

Enceinte , is a French term used technically in fortification for the inner ring of fortifications surrounding a town or a concentric castle....

.

Development

The relationship of the concentric castle to other forms of fortification is complex. The Roman and Byzantine castrum may be regarded a precursor, as its design also emphasized defence of the curtain wall and corner towers, as opposed to a keep.
Perhaps the earliest example of a concentric castle is the crusader castle of Belvoir

Belvoir Fortress (Israel)

Belvoir Fortress is a Crusader fortress in northern Israel, on a hill south of the Sea of Galilee. Gilbert of Assailly, Grand Master of the Knights Hospitaller, began construction of the castle in 1168. The restored fortress is located in Belvoir National Park...

, whose very regular rectangular layout has been described by Nicolle as one castrum nested inside another.
Historians (in particular Hugh Kennedy) have argued that the concentric defence arose as a response to advances in siege

Siege

A siege is a military blockade of a city or fortress with the intent of conquering by attrition or assault. The term derives from sedere, Latin for "to sit". Generally speaking, siege warfare is a form of constant, low intensity conflict characterized by one party holding a strong, static...

 technology in the crusader states from the 12th to the 13th century. The outer wall protected the inner one from siege engine

Siege engine

A siege engine is a device that is designed to break or circumvent city walls and other fortifications in siege warfare. Some have been operated close to the fortifications, while others have been used to attack from a distance. From antiquity, siege engines were constructed largely of wood and...

s, while the inner wall and the projecting towers provided flanking fire from crossbows. In addition, the strong towers may have served as platforms for trebuchet

Trebuchet

A trebuchet is a siege engine that was employed in the Middle Ages. It is sometimes called a "counterweight trebuchet" or "counterpoise trebuchet" in order to distinguish it from an earlier weapon that has come to be called the "traction trebuchet", the original version with pulling men instead of...

s for shooting back at the besiegers. The walls typically include tower

Tower

A tower is a tall structure, usually taller than it is wide, often by a significant margin. Towers are distinguished from masts by their lack of guy-wires....

s, arrow slits, and wall-head defenses such as crennelation and, in more advanced cases, machicolation

Machicolation

A machicolation is a floor opening between the supporting corbels of a battlement, through which stones, or other objects, could be dropped on attackers at the base of a defensive wall. The design was developed in the Middle Ages when the Norman crusaders returned. A machicolated battlement...

s, all aimed at an active style of defence. The Krak des Chevaliers

Krak des Chevaliers

Krak des Chevaliers , also Crac des Chevaliers, is a Crusader castle in Syria and one of the most important preserved medieval castles in the world. The site was first inhabited in the 11th century by a settlement of Kurds; as a result it was known as Hisn al Akrad, meaning the "Castle of the...

 in Syria

Syria

Syria , officially the Syrian Arab Republic , is a country in Western Asia, bordering Lebanon and the Mediterranean Sea to the West, Turkey to the north, Iraq to the east, Jordan to the south, and Israel to the southwest....

 is the best preserved of the concentric crusader castles. By contrast, Château Pèlerin

Chateau Pelerin

Château Pèlerin, also known as Atlit Castle and Castle Pilgrim, is located on the northern coast of Israel about south of Haifa. The Knights Templar began building it in 1218 during the Fifth Crusade. One of the major Crusader fortresses, it could support up to 4,000 troops in siege conditions. ...

 was not a concentric castle, as the side facing the sea did not require defensive walls. However, the two walls facing the land are built on the same defensive principles as other crusader castles in the same period, rivalling the defences at Krak.

While a concentric castle has double walls and towers on all sides, the defences need not be uniform in all directions. There can still be a concentration of defences at a vulnerable point. At Krak des Chevaliers, this is the case at the southern side, where the terrain permits an attacker to deploy siege engines. In addition, the gate and postern

Postern

A postern is a secondary door or gate, particularly in a fortification such as a city wall or castle curtain wall. Posterns were often located in a concealed location, allowing the occupants to come and go inconspicuously. In the event of a siege, a postern could act as a sally port, allowing...

s are typically strengthened using a bent entrance

Bent entrance

A bent entrance is a defensive feature in mediaeval fortification. In a castle with a bent entrance, the gate passage is narrow and turns sharply. Its purpose is to slow down attackers attempting to rush the gate and impede the use of battering rams against doors...

 or flanking towers.

Concentric castles were expensive to build, so that only the powerful military orders, the Hospitallers and Templars, or kings such as Edward I, could afford to build and maintain them. It has also been pointed out that the concentric layout suited the requirements of military orders such as the Hospitallers in resembling a monastry and housing a large garrison. Such castles were beyond the means of feudal barons. Thus concentric castles coexisted with simpler enclosure castles and tower keep

Keep

A keep is a type of fortified tower built within castles during the Middle Ages by European nobility. Scholars have debated the scope of the word keep, but usually consider it to refer to large towers in castles that were fortified residences, used as a refuge of last resort should the rest of the...

s even in the crusader states.

Concentric castles appeared in Europe

Europe

Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...

 in the 13th century, with the castles built in Wales

Wales

Wales is a country that is part of the United Kingdom and the island of Great Britain, bordered by England to its east and the Atlantic Ocean and Irish Sea to its west. It has a population of three million, and a total area of 20,779 km²...

 by Edward I providing some outstanding examples, in particular Beaumaris Castle

Beaumaris Castle

Beaumaris Castle, located in the town of the same name on the Isle of Anglesey in Wales, was built as part of King Edward I's campaign to conquer the north of Wales. It was designed by James of St. George and was begun in 1295, but never completed...

, a "perfect concentric castle", albeit unfinished. As Beaumaris was built on flat terrain rather than a spur, it was both necessary and possible to build walls and towers facing in all directions, giving a very regular, almost square, floor plan to the castle. Some influence from crusader fortification has been conjectured, but the amount of technology transfer from the East and much earlier Byzantine examples remains controversial among historians.

Similar structures

In German-speaking countries in the Holy Roman Empire

Holy Roman Empire

The Holy Roman Empire was a realm that existed from 962 to 1806 in Central Europe.It was ruled by the Holy Roman Emperor. Its character changed during the Middle Ages and the Early Modern period, when the power of the emperor gradually weakened in favour of the princes...

, many castles had double curtain walls with a narrow ward between them, referred to as a Zwinger. These were added at vulnerable points like the gate, but were rarely as fully developed as in the concentric castles in Wales or the Crusader castles.

The principle of an outer and inner wall was also used in fortified cities, such as the Theodosian Walls of Constantinople

Constantinople

Constantinople was the capital of the Roman, Eastern Roman, Byzantine, Latin, and Ottoman Empires. Throughout most of the Middle Ages, Constantinople was Europe's largest and wealthiest city.-Names:...

 and the city wall of Carcassonne

Carcassonne

Carcassonne is a fortified French town in the Aude department, of which it is the prefecture, in the former province of Languedoc.It is divided into the fortified Cité de Carcassonne and the more expansive lower city, the ville basse. Carcassone was founded by the Visigoths in the fifth century,...

.

The concept of mutually reinforcing lines of defence with flanking fire was continued in later periods, such as the early modern fortifications of Vauban

Vauban

Sébastien Le Prestre, Seigneur de Vauban and later Marquis de Vauban , commonly referred to as Vauban, was a Marshal of France and the foremost military engineer of his age, famed for his skill in both designing fortifications and breaking through them...

, where outer defence works were protected and overlooked by others and their capture did not destroy the integrity of the inner citadel

Citadel

A citadel is a fortress for protecting a town, sometimes incorporating a castle. The term derives from the same Latin root as the word "city", civis, meaning citizen....

.