thumb|300px|[[Krak des Chevaliers (Syria), the best preserved of the concentric crusader castles]]
thumb|Plan of [[Belvoir Fortress (Israel)|Belvoir Castle (Israel)]]
A concentric castle is a castle with two or more concentric curtain walls, such that the outer wall is lower than the inner and can be defended from it. The layout was square (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
thumb|Star-shaped plan of the citadel of [[Lille (France), designed by Vauban]]
thumb|right|Plan of [[Beaumaris Castle (Wales)]]
Surrounding fortresses or towns with a series of defensive walls 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 Assyrians, Persians, Ancient Egyptians and Babylonians. The ancient city of Lachish, a place in Israel, 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 Walls of Constantinople, which featured double layers of walls through most of its perimeter and a moat. The city of ancient Babylon also featured multiple layers of fortifications, famously seen in the Ishtar Gate. 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 Hugh Kennedy) have argued that the concentric defence arose as a response to advances in siege technology in the crusader states from the 12th to the 13th century. The outer wall protected the inner one from siege engines, while the inner wall and the projecting towers provided flanking fire from crossbows. Also, the strong towers may have served as platforms for trebuchets for shooting back at the besiegers. The walls typically include towers, arrowslits, and wall-head defences such as crenellation and, in more advanced cases, machicolations, all aimed at an active style of defence.
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.