I've heard that the Russian navy built some round warships from 1871 for use as a platform for artillery in shallow waters. They were used briefly in the Russo-Turkish War of 1877-78, but designated them storeships in 1903 and scrapped nine years later. For plans and photos http://www.futilitycloset.com/2012/07/20/round-numbers-4/.
Does anybody know if the British ever considered such a concept, built prototypes or went further? If so, I'd be intrigued to hear the details.