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 posterns 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 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 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 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, 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 the German-speaking states of the Holy Roman Empire, 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 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. Citadels from before and during the Reconquista in Spain and