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