Formerly a tidal island, 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. 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 during the period of the Napoleonic Wars.
The Castle incorporates the following museums:
- The Story of Castle Cornet
- Maritime Museum
- 201 Squadron RAF Museum
- Royal Guernsey Militia Museum - including artefacts from Royal Guernsey Light Infantry
It also has a restaurant, and hosts outdoor theatre performances during the summer months.