On March 8, 1803, a man named John Ericsson designed the first battleship with a revolving armored gun turret. Up to this point, the cannons on ships pointed pretty much in one direction. You had to turn the whole ship to point the guns.
Ericsson went to London in 1826 and designed and built many mechanical devices: a locomotive engine, caloric engines, and a screw propeller that affected navigation. Ericsson worked on a plan for marine engines that could go under a ship's waterline. He went to the United States in 1839. He lived there for the rest of his life, where he became a citizen in 1848.
In 1861, Ericsson designed for the United States Navy a ship that had his revolving gun-turret. Early in 1862, the Monitor was launched. March 9, 1862, the Monitor was involved in the first naval engagement between ironclad warships. The Confederate ship had been called the Merrimack when it was a Union ship. The Confederates had raised the ship from the Norfolk Naval Yard (where it had been sunk by the fleeing Union forces), put armor on it, and called it the Virginia.
The battle between the Virginia and the Monitor ended in a draw, with neither ship suffering heavy damage.
You can see several pictures of ironclads, courtesy of the National Archives, on our Ironclad Pictures page (the files are fairly large and can take from three to six minutes to download).
You can see a map of where the Monitor sank via our link to the History Network's page by clicking here.