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.

[1]: 2  The wardenship of Geoffrey de Lucy (1225–26) has been identified as a time of fortification in the Channel Islands: timber and lead was sent from England for castle building in Guernsey and Jersey.

[3] The island was retaken in 1340 and the castle was recaptured in August 1345[1]: 313  after a three-day attack by professional soldiers and the local militia.

[2] In 1358 the French returned and again captured the castle, but they were evicted the following year and an island traitor was executed.

[2] In 1372 Owain Lawgoch, a claimant to the Welsh throne, leading a free company on behalf of France, attacked Guernsey in an assault popularly called "La Descente des Aragousais".

Owain Lawgoch withdrew after killing 400 of the island militia, but without capturing the besieged Castle Cornet,[4] which he found strong and well supplied with artillery.

[2] The construction costs for works, repairs, maintenance, and the garrison were met from revenues raised in the island by the Warden (sometimes called "Keeper of the Castle") under royal warrant.

In 1547 the French, having captured Sark, descended on Guernsey; they met with gunfire from off St Peter Port and bombardment from cannon at the Castle.

In 1594 the "Royal Battery" was completed, as was the Sutlers house, and bastions of improved, polygonal form were constructed.

In 1627 King Charles I reduced the Crown's cost of running Castle Cornet by granting additional rights to Guernsey in a charter, in return for which the island became responsible for supplying victuals to the castle, including annual amounts of 100 tuns (1 tun holds 252 gallons) of beer, 600 flitches of bacon, 1,200 pounds of butter, 20 whey (around 4,600 pounds) of cheese, 3,000 stockfish, 300 pounds of tallow, twelve bulls, wood and coal.

[6] Throughout the siege, the Castle cannon fired on the town of St Peter Port, reducing many buildings and forcing the Royal Court to relocate to Elizabeth College.

While there, he recovered the Crown of England that had belonged to Charles I from the Court House in Jersey and brought it back to Guernsey, delivering it to the Governor of Castle Cornet.

[1]: 162  Two years after the execution of Charles I, while under the command of Colonel Roger Burges, the Castle surrendered on 17 December 1651.

[8] The Citadel which had yet another portcullis and door at the entrance and there are considerable works dating from the 1940-45 German occupation period.

[8] At the top there used to exist the round tower that was destroyed in the explosion of 1672 that killed seven including the Governors wife, Lady Hatton and some children.

Inside, it incorporates the following four additional museums: Every day (except for a few months in winter) at noon, a cannon is fired.

There is a restaurant, four period gardens, guided tours and the castle hosts outdoor theatre performances during the summer months.