other parts of the ship, the armor of the citadel could be made thicker. Save for the turrets, the ammunition hoists, the conning tower and part of the steering gear, nothing in the way of armor protected the remainder of the ship. When battle stations were called, the whole crew retreated into this area behind armored bulkheads and armored, watertight doors. The citadel can be visualized as an open-bottomed (closed top) rectangular armored raft with sloped sides sitting within the hull of the ship. From the box, shafts known as barbettes would lead upwards to the ship's main gun turrets and conning tower. Although it was desirable for the citadel to be as small as possible, the space enclosed was an important source of reserve buoyancy and helped prevent the ship from foundering when other compartments had flooded. Through compartmentalization and the redundancy of key systems, any damage done to the ship outside this armored box would likely be survivable. As long as those systems within the box remained intact, the ship could continue to fight. In effect, the scheme accepted vulnerability to medium-caliber and high-explosive shells striking the unarmored sections of the hull, in order to improve resistance against large-caliber armor-piercing shells without increasing the overall weight of armor. The unarmored parts of the ship would not offer enough resistance to armor-piercing shells to trigger their firing mechanisms (designed to explode after penetrating armor) so the shells would pass through without exploding, while the vital parts could have armor thick enough to resist the heaviest shells. To maximize the thickness of armor available for a given weight, it was desirable that the citadel be as small as possible. One way to achieve this was to concentrate the main battery in three turrets of triple or even two turrets of quadruple (quad) gun mountings, as opposed to four twin turrets typical during the First World War. In some cases, the turrets had an all-forward layout, such as the Royal Navy's and the French Navy's . Another way is more compact and efficient machinery such as the French navy's use of "supercharged" Indret boilers for the Dunkerque class or the US Navy's decision to combine double-reduction gear turbines with extreme steam conditions (ultra high heat and pressure) in the , and . Evolution The majority of the battleships up through World War I vintage had armor disposed in belts of varying