Thick-Armored.

Louis Prang & Co., The Monitor and Merrimac: The First Fight Between Ironclads, 1886

Right down the street from my sis' house, on this day in 1862, the Monitor and the Merrimack buttoned up and did battle on the Elizabeth River in Virginia.

American Battlefield Trust has the details ...

The CSS Virginia was fairly conventional. Built upon the hull of the USS Merrimac, it was a wooden vessel covered with iron plates, and it had fixed weapons. Still, she was a formidable threat. Iron covered, the ship measured 275 feet long 38.5 feet across its beam, and 27.5 feet deep. It was angled such that cannon shot would harmlessly bounce off its sides. Outfitted with ten guns and resembling a floating barn roof, the ship was rechristened the CSS Virginia and released from dry dock into the Elizabeth River on February 17, 1862.

The ship possessed a mixed armament consisting of two 7-in. Brooke rifles, two 6.4-in. Brooke rifles, six 9-in. Dahlgren smoothbores, as well as two 12-pdr howitzers. While the bulk of the guns were mounted in the ship's broadside, the two 7-in. Brooke rifles were mounted on pivots at the bow and stern and could traverse to fire from multiple gun ports. In creating the ship, the designers concluded that its guns would be unable to penetrate the armor of another ironclad. As a result, they had Virginia fitted with a large ram on the bow. Though a formidable vessel, Virginia's size and balky engines made it difficult to maneuver and complete circle required a mile of space and forty-five minutes. 

When federal authorities discovered in the summer of 1861 that the Confederates were armoring the old Merrimac, they knew they had to commission a unique vessel of their own to challenge her.  USS Monitor, was commissioned on February 25, 1862 at New York City, New York.   An innovative warship, she had a thick-armored round turret which was twenty-feet in diameter.  Rotated by steam power, the turret could fire nearly 360 degrees from a pair of eleven-inch Dahlgren smoothbore shell guns.

The Monitor had a round, rotating gun turret on her deck. The iron turret housed two Dahlgren guns, installed side by side. Innovative in design as well as in construction, the Monitor had components and attributes that were later echoed in submarine design. Her armored deck, 172 feet long and 41.5 feet at the beam, rode just 18 inches above the waterline and extended beyond the hull, which was only 5/8-inch thick and thus nearly submerged for protection against cannon fire. On the deck with the gun turret was a small pilothouse and a detachable smokestack. The turret was made from eight layers of 1-inch iron plate bolted together. A ninth plate within the turret provided a sound barrier.