Castle Cornet is a large island castle in Guernsey, and former tidal island, also known as Cornet Rock or Castle Rock, which has been part of one of the breakwaters of St Peter Port's harbour, the main one in the island, since 1859. The island measures about two hectares in area, with a length of 175 metres and a width of 130 metres. It lies not quite 600 metres east of the coast of Guernsey. Formerly a tidal island, like Lihou on the west coast of Guernsey, it 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, 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. It 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.