Concentric castle



A is a  with two or more concentric s, such that the outer wall is lower than the inner and can be defended from it. The layout is typically square (at and ) where the terrain permits, or an irregular polygon (at  and ) where curtain walls of a  follows the contours of a hill.

Concentric castles resemble one castle nested inside the other, thus creating an or. They are typically built without a central free-standing. Where the castle includes a particularly strong tower (donjon), such as at Krak or Margat, it projects from the inner.

Development
Surrounding fortresses or towns with a series of s where the outer walls are lower than the inner walls is something that has been found in fortifications going back thousands of years to cultures like the, , and. The ancient city of, a place in , was excavated and found to consist of multiple walls that were illustrated in Assyrian art documenting their successful siege of the city. The Byzantines also famously constructed the, which featured double layers of walls through most of its perimeter and a. The city of ancient Babylon also featured multiple layers of s, famously seen in the. However, the relationship of the concentric castle to other forms of fortification is complex. An example of an early concentric castle is the Byzantine castle of Korykos in Turkey, built in the early 11th century AD.

Historians (in particular ) have argued that the concentric defense arose as a response to advances in technology in the  from the 12th to the 13th century. The outer wall protected the inner one from s, while the inner wall and the projecting towers provided flanking fire from. Also, the strong towers may have served as platforms for s for shooting back at the besiegers. The walls typically include s, s, and wall-head defences such as and, in more advanced cases, s, all aimed at an active style of defence. The in  is the best-preserved of the concentric crusader castles. By contrast, 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. Also, the gate and s are typically strengthened using a or flanking towers.

Concentric castles were expensive to build, so that only the powerful military orders, the and, or powerful kings 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 and housing a large garrison. Such castles were beyond the means of feudal barons. Thus, concentric castles coexisted with simpler enclosure castles and tower s even in the crusader states.

Concentric castles appeared in in the 13th century, with the castles built in  by  providing some outstanding examples, in particular, a “perfect concentric castle”, albeit unfinished. As Beaumaris was built on flat terrain rather than a, 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 the German-speaking states of the, many castles had double curtain walls with a narrow ward between them, referred to as a. 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 of  and the city wall of.

The concept of mutually reinforcing lines of defense with flanking fire was continued in later periods, such as the early modern fortifications of, where outer defense works were protected and overlooked by others and their capture did not destroy the integrity of the inner.

Citadels from before and during the Reconquista in Spain and Portugal also have fortifications similar to concentric castles found elsewhere in Europe. is a good example of such a fortress along with in  Portugal.

Examples of concentric castles
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