The reduit (from French réduit) is a reinforced defensive structure that served as a retreat for the crew if the defensive wall in front was overcome by the enemy. The reduit (the core work) is located inside a defensive wall. They were increasingly used in the detached works of the New Prussian/New German fortification style of the 19th century. They were among the strongest fortifications and were intended to ensure a stubborn defense. The citadel, for example, is the reduit or retreat within a fortress, analogous to this, the keep in castle construction had the function of a reduit.