Formerly a tidal island, it was originally fortified until the 13th century. In 1066 when William Duke of Normandy became William I, King of England, the island became possession of the English Crown. In 1204, King John lost control of Normandy but the island remained in the possession of the English Crown.
In 1338 the French attacked and occupied the castle, which they held until 1345, where it was recaptured by the English. In 1358 the French returned and recaptured the castle, but were expelled the following year. French attacks lasted until the 16th century.
Castle Cornet was upgraded many times, with constructions and additions, due to the serious damage it suffered during the conflicts with the French and the English Civil Wars. Many modifications were made to the castle during World War II by the German occupiers as the defenses had to be updated again for the modern war.
The castle houses five museums. These include The Story of Castle Cornet, the 201 Squadron (RAF) Museum, the Naval Museum, the Royal Guernsey Light Infantry Museum and the Royal Guernsey Militia Museum. Also, within its walls there are four historic gardens with 17th and 19th century motifs which are taken care of by the castle owners.