Concentric Castles and Crusader Castles

A concentric castle is a castle with two or more concentric curtain walls, where the outer wall is lower than the inner and can be defended from it.

Concentric castles resemble one enclosure castle nested inside the other, generally without a central free-standing keep. Where the castle includes a particularly strong tower (donjon), such as at Krak or Margat, it projects from the inner enceinte providing a sort of citadel - a castle within a castle within a castle.

The word concentric does not imply that these castles were circular in plan. The layout was rectangular where the terrain permitted (Belvoir, Beaumaris), or an irregular polygon where curtain walls of a spur castle followed the contours of a hill (Krak, Margat).

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 emphasised defence of the curtain wall and corner towers, as opposed to a keep as favoured by the Normans.

In German-speaking countries, many castles had double curtain walls with a narrow ward between them, called a Zwinger (English lists, French lices). These double walls were usually added at vulnerable points like the gate, but rarely as fully developed as the in concentric castles.

The concentric design may well have originated in the Crusader states. The earliest example of a concentric castle is the crusader castle of Belvoir (c 1168), whose regular rectangular layout has been described as one castrum nested inside another. Some historians have plausibly argued that the concentric defence arose as a response to advances in siege technology in the crusader states from the 12th to the 13th centuries.

In a concentric castle the outer wall protected the inner one from siege engines, while the inner wall and the projecting towers provided flanking fire from crossbows. In addition, the strong towers served as platforms for trebuchets for shooting back at the besiegers.

Walls typically include intramural towers, arrow slits, and wall-head defences such as crenellations or machicolations all aimed at an active style of defence. In addition, the gate and posterns are typically strengthened using a bent entrance with flanking towers. Krak des Chevaliers in Syria is the best preserved of the concentric crusader castles.

While a concentric castle has double walls and towers on all sides, the defences are not necessarily uniform in all directions. There can 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.

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.

The concentric layout particularly suited the requirements of military orders such as the Hospitallers in resembling a monastery and housing a large garrison of brothers. Such castles were beyond the means of feudal barons. Consequently, concentric castles coexisted with more modest enclosure castles and tower keeps even in the crusader states.

Concentric castles appeared in Europe in the 13th century, with the castles built in Wales by Edward I providing some outstanding examples, in particular Beaumaris Castle, although Beaumaris remains unfinished. As Beaumaris was built on flat terrain, it was necessary 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.

The principle of an outer and inner wall was also used in fortified cities, such as the Theodosian Walls of Constantinople and the city wall of Carcassonne. The concept of mutually reinforcing lines of defence with flanking fire was continued in later periods, such as the early modern fortifications of de Vauban, where outer defence works were protected and overlooked by others and their capture did not destroy the integrity of the inner citadel.

Beaumaris Castle in Wales Photograph		Beaumaris Castle in Wales sketch of how it would have looked if completed

Photographs of Concentric Design

The principles of concentric design can be seen in castles that are not technically Concentric Castles.

English Device Forts

In 1534 the Act of Supremacy passed by the Parliament of England made Henry VIII the head of the English church, challenging the authority of the papacy. When Pope Paul III negotiated a peace in 1538 between the Holy Roman Emperor (Charles), and the King of France (Francis I). The new peace represented a threat to England, In response Henry undertook a programme of fort-building along England's south coast.

In the first phase of Henry’s fortification programme, 30 castles and forts were built. A curvilinear form of fortification was developed which consisted of squat artillery towers surrounded by round bastions to provide platforms for artillery. Good examples of these are Walmer Castle and Deal Castle in Kent, Calshot Castle and Hurst Castle in Hampshire and St Mawes Castle in Cornwall.

A new programme was begun after renewed threat from the French in the 1540s. This concentrated on the vulnerable area around the Solent estuary and the ports of Southampton and Portsmouth. This programme of coastal defences reflected new developments in fortifications, notably in the introduction of Italianate angular features. Earlier curvilinear designs of castles were now replaced by square keeps surrounded by angular or arrow-head shaped bastions. Good examples of these are Yarmouth Castle and Southsea Castle built around 1545. These represented a transitionary form between Henry's round forts and so-called "Star Forts".

Camber Castle Sussex 1513–43
Worsleys castle Isle of Wight 1522–5
Little Dennis Blockhouse Cornwall 1537-40
Devils Point Artillery Tower Devon 1537–39
St Catherine's Castle Cornwall 1538–40
Southsea Castle Hampshire 1538–44
Deal Castle Kent 1539
Walmer Castle Kent 1539
Gravesend Blockhouse Kent 1539
Higham Blockhouse Kent 1539
Milton Blockhouse Kent 1539
West Tilbury Blockhouse Essex 1539
Calshot Castle Hampshire 1539–40
Sandgate Castle Kent 1539–40
Sandown Castle Kent 1539–40
Portland Castle Dorset 1539–40
West Cowes Castle Isle of Wight 1539–40
East Tilbury Blockhouse Essex 1539–41
East Cowes Castle Isle of Wight 1539–42
St Helens Bulwark Isle of Wight 1539–45
St Mawes Castle Cornwall 1540–5
Pendennis Castle Cornwall 1540–5
Sandsfoot Castle Dorset 1541
Hurst Castle Hampshire 1541–44
Netley Castle Hampshire 1542–45
St Andrews Castle Hampshire 1543–44
Sandown Castle Isle of Wight 1545
Yarmouth Castle Isle of Wight 1545
Brownsea Castle Poole Harbour 1545–1547
Sharpenode Bulwark Isle of Wight 1545–1547

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