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.