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.
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.http://guernseymansions.blogspot.com/
