An afterthought from the 1859 Royal Commission on the Defence of the United Kingdom, Fort Borstal was built between 1875 and 1885 by convict labour to hold the high ground southwest of Rochester, Kent. A polygonal design, never armed, with a anti-aircraft battery in WWII. After many years use as a pig farm for the nearby Young Offenders Institution it was sold in 1991 to a company hoping to make it a museum, but that proved unsuccessful, so its likely to be used for accommodation now. There is no public access to the site.