Conning tower

A conning tower is a raised platform on a ship or submarine, often armored, from which an officer in charge can conn the vessel, controlling movements of the ship by giving orders to those responsible for the ship's engine, rudder, lines, and ground tackle. It is usually located as high on the ship as practical, to give the conning team good visibility of the entirety of the ship, ocean conditions, and other vessels.
The verb "conn" probably stems from the verb "conduct" rather than another plausible precedent, the verb "control".
Surface ships.
On surface ships, the conning tower was a feature of all battleships and armored cruisers from about 1860 to the early years of World War II. Located at the front end of the superstructure, the conning tower was a heavily armored cylinder, with tiny slit windows on three sides providing a reasonable field of view. Designed to shield just enough personnel and devices for navigation during battles, its interior was cramped and basic, with little more than engine order telegraphs, speaking tubes or telephones, and perhaps a steering wheel. At all other times than during battles, the ship would be navigated from the bridge.
Conning towers were used by the French on their floating batteries at the Battle of Kinburn. They were then fitted to the first ironclad the . The first Royal Navy (RN) conning tower appeared on which had 3 inches of armour.
In the Royal Navy, the conning tower became a massive structure reaching weights of hundreds of tons on the s (such as ), and formed part of a massive armoured citadel (superstructure) on the mid-1920s s, which had armour over a foot thick. The , in contrast to the "Nelson" class, had comparatively light conning tower protection with sides, front and rear, and roof and deck. The RN's analysis of World War I combat revealed that command personnel were unlikely to use an armoured conning tower, preferring the superior visibility of unarmoured bridge positions. Older RN battleships that were reconstructed with new superstructures had their heavily armoured conning towers removed and replaced with much lighter structures. These new conning towers were also placed much higher in the ship, for superior visibility. There is no evidence that RN captains and admirals used the armoured conning towers on those ships that did have them during World War II, with, for example, Vice-Admiral Holland and Captain Kerr commanding "Hood" during the Battle of the Denmark Strait from her unarmoured bridge. Even in the United States Navy (USN), battleship captains and admirals preferred to use the unarmoured bridge positions during combat.
The USN had mixed opinions of the conning tower, pointing out that its weight, high above the ship's center of gravity, did not contribute directly to fighting ability. Beginning in the late 1930s, as radar surpassed visual sighting as the primary method of detecting other ships, battleships began reducing or eliminating the conning tower. The battle of Guadalcanal during World War II briefly slowed this trend: when the hit on the superstructure, many exposed crewmen were killed or wounded; yet Admiral Lee and Captain Davis of declined to use the armoured conning tower during the battle. Soon the heavy battleship conning towers were removed from , , , and during their post-Pearl Harbor attack reconstructions and replaced with much lighter cruiser-style conning towers.
By the end of World War II, US ships were designed with expanded weather bridges enclosing the armored conning towers. On s, the conning tower is a thick vertical armor-plated cylinder with slit windows located in the middle of the bridge, climbing from deck 03 all the way up to the flying bridge on 05.
With the demise of battleships after World War II, along with the advent of missiles and nuclear weapons during the Cold War, modern warships no longer feature conning towers.
Submarines.
The conning tower of a pre-1950s submarine was a small watertight compartment mounted above the main pressure hull, equipped with instruments and controls and from which the periscopes were used to direct the boat and launch torpedo attacks. Above this was an open bridge for surface navigation and signalling, and the gun direction platform. It should not be confused with the submarine's control room, which was directly below it in the main pressure hull, or the bridge, which was a small exposed platform in the top of the sail. 
As improvements in technology allowed the periscopes to be made longer it became unnecessary to raise the conning station above the main pressure hull. (laid down 1956) was the last American submarine to have a conning tower. The additional conning tower pressure hull was eliminated and its functions were added to the command and control center. Thus it is incorrect to refer to the sail (or fin in British usage) of a modern submarine as a "conning tower".