An obelisk was erected on Bréhon island in 1744 to serve as a sea mark. However the lack of visibility of the obelisk led to its replacement in 1824 by a tower 40 ft high and 34 ft in circumference, topped by a globe. During the tenure (1803-1813) of Lieutenant Governor General Sir John Doyle, there were plans to erect a guardhouse on Bréhon, but nothing came of these. Doyle was responsible, however, for substantial fortification efforts elsewhere in Guernsey, including the construction of the Martello towers of Fort Grey, Fort Saumarez, and Fort Hommet.
In the 1840s there was a renewed concern with France, with particular concern for the protection of Alderney and the other Channel Islands because of their strategic importance in the Channel. Lieutenant Governor Major-General Sir William Francis Patrick Napier proposed a number of works, including the establishment of a fort on Bréhon. In 1850 the British became concerned that the French had created fortifications at Cherbourg. This led to the construction of several towers and forts in the Channel area. The Alderney cutter Experiment was wrecked off Bréhon in March 1850. A review of Guerney's defences in 1852 recommended the construction of three artillery barracks, Fort Richmond, Fort Hommet, and Fort Le Marchant, the upgrading of Fort Doyle, and the construction of Bréhon Tower.
Work on Bréhon Tower commenced in 1854 and was completed in 1856. Bréhon Tower's role was to guard the shipping channel between Guernsey and Herm, and help protect the harbour of St Peter Port. The fort's footprint measures 65 feet by 85 feet (at the widest point), and the tower stands 34 feet high. The tower has three levels. The magazine, shell room, shifting room, stores, and fresh-water cistern were all on the ground floor. It also had latrines on the same floor, a Victorian innovation. The first floor contained the garrison's living quarters. The original plan was to put three heavy guns on the gun platform at the top, with five guns on the (second) floor below, sharing the 14 cannon ports. However, during construction the armament was cut back to three 68-pounder and two 10-inch shell guns, all on the gun platform.
By World War I the tower was obsolete and the War Office turned it over to the States of Guernsey. The garrison was originally to have consisted of two officers and 60 NCOs and other ranks, but with the reduction in armament, two officers and 30 men was deemed sufficient. During World War II, the Germans placed an anti-aircraft gun on the tower. This is credited with shooting down several allied planes, and one German plane, which crashed on Crevichon.
The Bréhon Tower is accessible only by boat.
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