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Enter Rear-Admiral Popov, who was in charge of Russian shipbuilding. Somehow, perhaps from playing with Frisbees in his bath tub, he got it in his head that what was needed were round warships.
Six were planned, but only two were ever built. In the Russo-Turkish War of 1877 they finally had a chance to prove their worth. Needless to say, they were complete failures. They were too slow to pursue Turkish ships, and had too short a cruising range to even reach the Turkish coast to bombard it. In effect, they were little more than floating gun emplacements.
Above are pictures of the design and one of the ships, below are pictures of a model of one. The pictures are from the EnglishRussia post Round Ships of the Russian Admiral. That post has many more pictures, as well as quite a bit of commentary about the round ships' history.