A linear 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...

 that was designed to confront its attackers with a series of barriers/impediments in a line (hence the name).

The principle was to funnel assaulting forces into attacking a very narrow front and focusing all of the castle's defences in that area. This would mean that both architectural devices and manpower could be concentrated, unlike the more familiar concentric castle

Concentric castle

A concentric castle is a castle with two or more concentric curtain walls, such that the outer wall is lower than the inner and can be defended from it. The word concentric does not imply that these castles were circular; in fact if taken too literally the term "concentric" is quite misleading...

, for which the entire circumference needed to be defended.

Linear castles tended to be built where geography favoured this approach. The Knights Hospitaller

Knights Hospitaller

The Sovereign Military Hospitaller Order of Saint John of Jerusalem of Rhodes and of Malta , also known as the Sovereign Military Order of Malta , Order of Malta or Knights of Malta, is a Roman Catholic lay religious order, traditionally of military, chivalrous, noble nature. It is the world's...

 fortress of 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...

 in Syria is built on a narrow rocky promontory

Promontory

Promontory may refer to:*Promontory, a prominent mass of land which overlooks lower lying land or a body of water*Promontory, Utah, the location where the United States first Transcontinental Railroad was completed...

, ideal for preventing successful undermining but inappropriate for the rings of walls needed for a concentric castle. The core of 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...

 is located in a narrow gully carved from sheer rock walls severely limiting possibilities for a flank attack.

The theory of linear castles arrived in Britain in the mid thirteenth century, so the concept co-existed with concentric castles. The earliest recognised linear castle in Britain is at Tintagel

Tintagel Castle

Tintagel Castle is a medieval fortification located on the peninsula of Tintagel Island, adjacent to the village of Tintagel in Cornwall, United Kingdom. The site was possibly occupied in the Romano-British period, due to an array of artefacts dating to this period which have been found on the...

, having a series of defensive wards in a row. Conway and Caernarvon Castles were both built in a linear style, defending a peninsula

Peninsula

A peninsula is a piece of land that is bordered by water on three sides but connected to mainland. In many Germanic and Celtic languages and also in Baltic, Slavic and Hungarian, peninsulas are called "half-islands"....

.

It is also worth noting that many castles naturally combined elements of both concentricity and linearity. Conwy Castle

Conwy Castle

Conwy Castle is a castle in Conwy, on the north coast of Wales.It was built between 1283 and 1289 during King Edward I's second campaign in North Wales....

, often cited as a concentric castle, is really a linear castle with concentric elements bolted on.