Cliffe Fort is a Royal Commission fort built in the 1860s on the edge of the Cliffe marshes on the Hoo Peninsula in north Kent, England to protect against invasion via the river Thames. It is opposite Coalhouse Fort in Essex: they are 2 km apart. Construction was difficult due to the marshy ground and the malaria-carrying mosquitos. It was armed with 12.5-inch 38-ton and 11-inch 25-ton rifled muzzle-loading guns. Protection of these guns was provided by granite-faced casemates with shields for added defence.