Fort Darnet, like Fort Hoo

Fort Hoo

Fort Hoo, like Fort Darnet was built on the recommendations of the 1859 Royal Commission on an island covering the inner navigable channel of the River Medway, Kent.-History:...

 was built on the recommendations of the 1859 Royal Commission

Royal Commission on the Defence of the United Kingdom

In 1859 Lord Palmerston instigated the Royal Commission on the Defence of the United Kingdom because of serious concerns that France might attempt to invade the UK...

 on an island covering the inner navigable channel of the River Medway

River Medway

The River Medway, which is almost entirely in Kent, England, flows for from just inside the West Sussex border to the point where it enters the Thames Estuary....

, Kent

Kent

Kent is a county in southeast England, and is one of the home counties. It borders East Sussex, Surrey and Greater London and has a defined boundary with Essex in the middle of the Thames Estuary. The ceremonial county boundaries of Kent include the shire county of Kent and the unitary borough of...

.

Originally designed for two tiers of guns mounted in a circle, with a boom strung between them, there were many problems with subsidence, and after extensive cost overruns the forts were completed in 1871 with one tier, and 11 guns : a mixture of 9-inch

RML 9 inch 12 ton gun

The RML 9 inch guns Mark I - Mark VI were large rifled muzzle-loading guns used as primary armament on smaller British ironclad battleships and secondary armament on larger battleships, and also ashore for coast defence.-Design:...

  and 7-inch rifled muzzle-loaders, and no boom. The forts were never used in anger, and were disarmed before the First World War.

In the Second World War the fort was used as observation posts, with platforms and pillboxes

British anti-invasion preparations of World War II

British anti-invasion preparations of the Second World War entailed a large-scale division of military and civilian mobilisation in response to the threat of invasion by German armed forces in 1940 and 1941. The British army needed to recover from the defeat of the British Expeditionary Force in...

built on top. The fort is still in fair condition, however the magazine level seems to have been deliberately flooded to minimise access and vandalism. The island can be freely visited by boat, though the landing is muddy.