"Monitors" are a family of ship named for the first of the class to be completed and the prototype for the concept, USS Monitor , designed by John Ericsson , Swedish inventor and US Citizen. The monitor type was first used as a warship during the American Civil War, but remained a feature of naval warfare until the last vessels of the design were decommissioned in 1950s. Similar vessels designed for riverine services were built later, most notably during the Vietnam War, but these bore little resemblance to the original concept.
Ericsson's vision called for an armored warship maximizing firepower while minimizing its own surface to enemy fire. To that end, Ericsson developed a design which placed a pair of turreted guns atop a flat, raft like hull, originally constructed with no vertical surfaces above the waterline with the exception of the turret and a slightly raised pilot house to the fore of the hull. The initial prototype, USS Monitor, was described famously in one source as "looking like a cheesebox on a raft." Wartime experience with Monitor gradually exposed the weaknesses in the design, and in addition to raised, retractable stacks, further designs placed the pilothouse atop the turret, to improve coordination and allow firing directly to the fore. In addition, the design was less than acceptable in heavy seas, with a tendency to take on a great deal of water due to very little freeboard; subsequent designs greatly improved this, but monitors were, for the most part, never renown for their seakeeping abilities, and even so-called "Ocean Going" Monitors had their own, unique issues.
Several experiments were attempted to increase firepower during the ACW period, and included the addition of one or two turrets. Some designs called for even more turrets, but were never completed. The most heavily armed monitor to be completed during the war was USS Roanoke , carrying three turrets. However, Roanoke was widely considered a design and engineering failure, and the experiment was not repeated by the US Navy during the war.
After the war, seakeeping qualities of monitors were greatly improved, but they continued to be regarded as coastal and riverine defense vessels rather than true oceanic warships till the last of their kind were decommissioned.
The Union built all of the monitors built to serve in the United States during the 1861-1865 period. The Confederacy considered turreted monitor designs, but money and resources were allocated for only a single example - CSS Columbus - which was never completed. The principal reason for this was that armored casemate style ironclad warships were simply easier to construct with the limited industrial resources available to the South than the relatively complex mechanisms required to construct and properly operate a turret. However, it is worth mentioning that some indigenous proposals for less complicated monitors were apparently proposed, though the vast majority were not, it would seem, considered with any degree of seriousness. Money was not the issue, so much as supply, however, and so the Confederacy did attempt to buy turreted warships over the course of the war. Of the turreted vessels purchased for use by the CSN from foreign builders, only one warship, CSS Stonewall actually served under Confederate colors, and she was not a monitor. At least one post-war apologist has argued that the South's needs might have been better served by Monitor type designs.