Fort Borstal was built as an afterthought from the 1859 Royal Commission on the Defence of the United Kingdom, by convict labour between 1875 and 1885, to hold the high ground southwest of Rochester, Kent. It is of polygonal design and was never originally armed. An anti-aircraft battery was based there in the Second World War.
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 and the fort has been converted into living accommodation. There is no public access to the site.
Coordinates: 51°22′14″N 00°29′21″E / 51.37056°N 0.48917°E / 51.37056; 0.48917