Concentric castle A concentric castle is a castle with two or more concentric curtain walls , such that the inner wall is higher than the outer 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 . The layout was rectangular ( at Belvoir and Beaumaris ) where the terrain permitted , or an irregular polygon ( at Krak and Margat ) where curtain walls of a spur castle followed the contours of a hill . Concentric castles resemble one castle nested inside the other , thus creating an inner and outer ward . They are typically built 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 . Development . The relationship of the concentric castle to other forms of fortification is complex . Perhaps the earliest example of a concentric castle is the crusader castle of Belvoir , 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 technology in the crusader state s from the 12th to the 13th century . The outer wall protected the inner one from siege engine 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 s for shooting back at the besiegers . The walls typically include towers , arrow slits , and wall - head defenses such as crenellation and , in more advanced cases , machicolation s , all aimed at an active style of defence . The Krak des Chevaliers in Syria is the best preserved of the concentric crusader castles . By contrast , Château Pèlerin 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 s are typically strengthened using a bent entrance 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 monastery 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 s even in the crusader state s . 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 , 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 , 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 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 Vauban , where outer defence works were protected and overlooked by others and their capture did not destroy the integrity of the inner citadel .