The Bréhon Tower (Fort Brehon) is accessible only by boat and sits on Bréhon Rock, an island in the Little Russell. The stretch of water between St Peter Port and Herm.
In 1744 a sea mark was established on Brehon Rock but after complaints were received that it was not very obvious it was replaced by a 40 foot mast.
At the start of the 19th Century a report was submitted to the Lieutenant Governor General Sir John Doyle on building a guardhouse on the Brehon Rock to defend against a French invasion.
The report suggested a guardhouse which could double as a signal station would be a wooden building, 14 feet square, with a stove or fireplace and a cellar for provisions and water.
It would include a signal mast, a small cannon for signaling in mist, a speaking trumpet, a small boat and it was expected to cost £100 (about £9,000 today), but nothing was built.
In 1850 it came to the attention of the British that the French had created fortifications at Cherbourg and this prompted the building of defences at Portland, Jersey, Alderney and the work on Brehon.
The tower replaced a pyramid sea marker on the rock and was completed in 1856 as Britain entered a period of peace with the end of the Crimean War.
With the threat of invasion from France lessening towards the end of the 19th Century the cannon were removed and the tower changed ownership from the War Office to the States of Guernsey in 1914.
When the Germans occupied the islands during the Second World War they put a small garrison on Brehon with an anti-aircraft gun on the top.
This gun shot down several planes including one of its own as a Focke-Wulf FW190 was shot down east of Herm.
The last time the tower was garrisoned was for one day in January 1946. A small party of British soldiers and prisoners of war had been ferried over to remove the remaining German ammunition but they ended up spending the night as bad weather stranded them.
Although not strictly a Martello tower, Bréhon represents the final evolution of the basic design of the Martello tower.
Today, although the site is open to visitors, the tower is closed. The tower holds a light operated by the Guernsey Harbour Authority. The island is home to a breeding colony of common terns