separate protected positions for field guns; these concrete structures were copied by the French who called them casemates de Bourges (French article: Casemate de Bourges) after the proving ground where they had been tested. 20th century Following experience gained in the World War I, French engineers began to design a new scheme of fortifications to protect their eastern border, which became known as the Maginot Line. The main element of this line were large underground forts based on the feste principle, whose main armament was in turrets, however the countryside between them was defended by smaller self-sufficient works based on the earlier casemates de bourges, housing either light field guns or anti-tank guns. As the World War II approached, similar casemate designs were adopted by other European nations as they offered protection from attacking aircraft. The German Organisation Todt undertook the development of casemates for thé large coastal guns of the Atlantic Wall. Built of concrete up to thick, they were thought to be able to withstand any form of attack. Work by the Western Allies to develop countermeasures that could defeat casemates and other types of bunker resulted in weapons such as tank-mounted spigot mortars, rocket-assisted projectiles, recoilless rifles, various types of demolition charge and earthquake bombs. Naval In warship design the term "casemate" has been used in a number of ways, but it generally refers to a protected space for guns within a ship's hull or superstructure. The first ironclad warship, the (1858), was a wooden steamship whose hull was covered with armored plating, tested to withstand the largest smoothbore guns available at the time. The response by the British Royal Navy to this perceived threat was to build an iron-hulled frigate, . However, it was realised that to armor all of the hull to fully withstand the latest rifled artillery would make it unfeasibly heavy, so it was decided to create an armored box or casemate around the main gun deck, leaving the bow and stern unarmored. The American Civil War saw the use of casemate ironclads: armored steamboats with a very low freeboard and their guns on the main deck ('Casemate deck') protected by a sloped armoured casemate, which sat atop the hull. Although both sides of the Civil War used casemate ironclads, the ship is mostly associated with the southern Confederacy, as the north also employed turreted monitors, which the south was unable to produce.