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. 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.