Formerly a tidal island, Cornet Rock was first fortified as a castle between 1206 and 1256, following the division of the Duchy of Normandy in 1204. In 1339 when a French force captured the island and occupied it for several years, Cornet was besieged and captured, and the garrison massacred. With the advent of cannon and gunpowder, the castle was remodelled between 1545 and 1548. Prof. John Le Patourel, in The Building of Castle Cornet mentions that in 1566, iron and hammers were taken to 'Creavissham' (i.e. Crevichon), and that island quarried for the castle.
Castle Cornet served as official residence of the Governor of Guernsey until 1672 when the keep was catastrophically destroyed. A bolt of lightning struck the magazine of the castle, destroying the keep and a number of living quarters. The Governor at the time was Lord Hatton. His mother, wife and a number of members of staff were killed in the explosion. It became integrated into the breakwater after the period of the Napoleonic Wars. Along the breakwater, a pond for toy yachts was constructed in 1887 for Queen Victoria's Diamond Jubilee, which served as a seaplane base during World War I. In World War II, it became occupied by a small group of German troops. It was presented to the people of Guernsey in 1945 by the Crown.
The castle incorporates today several museums, like castle museum, maritime museum and Royal Guernsey Militia museum.
References:The Broch of Clickimin is a large and well preserved, though somewhat restored broch near Lerwick. Originally built on an island in Clickimin Loch (now increased in size by silting and drainage), it was approached by a stone causeway. The water-level in the loch was reduced in 1874, leaving the broch high and dry. The broch is situated within a walled enclosure and, unusually for brochs, features a large 'blockhouse' between the opening in the enclosure and the broch itself. Another unusual feature is a stone slab featuring sculptured footprints, located in the causeway which approached the site. Situated across the loch is the Clickimin Leisure Centre.