Castle Cornet sits 600 meters east of Guernsey's coast, a flattened oval of rock 175 meters long and 130 meters wide. Until 1859, when it became part of the breakwater of St Peter Port harbor, it was a true tidal island - cut off twice a day by the rising sea. Fortification began somewhere between 1206 and 1256, in the aftermath of the 1204 split that left Guernsey loyal to England. Over the next 800 years, generation after generation added walls, gateways, batteries, and bastions. There are six gateways now between the harbor and the citadel at the top, and almost no order to how any of it was built. The main gate is hidden from the line of fire from the island. Above it sits the coat of arms of Queen Elizabeth I.

In 1338, in the opening years of what would become the Hundred Years' War, a French force seized Guernsey and besieged Cornet. The castle fell on 8 September. The French then massacred the garrison - eleven men-at-arms and fifty archers, killed after surrender. Guernsey was retaken by English forces in 1340, but the castle itself remained in French hands until August 1345, when an English commander recaptured it and found it, despite everything, strong and well stocked with artillery. The medieval island had absorbed the worst of war and held.

When the English Civil War broke out in 1642, Guernsey overwhelmingly sided with Parliament. The garrison of Castle Cornet did not. Sir Peter Osborne, the King's governor, refused to surrender. For nine years his small force held out, supplied by sea from Royalist Jersey, while Parliamentary cannon thundered at them from the town and the castle's own guns thundered back. The cannonballs - ten thousand of them by contemporary estimates - fell on St Peter Port itself, where Guernseymen who supported Parliament watched their own neighbors' homes destroyed by their own would-be defenders. Castle Cornet finally surrendered on 17 December 1651, one of the very last Royalist positions in the British Isles to hold out. Its commander, Colonel Roger Burges, walked out with honor intact.

Then came 1672. On the night of 29 December, lightning struck the powder magazine inside the castle's medieval keep, and the keep itself simply ceased to exist in the explosion that followed. A 1672 engraving made before the disaster shows the tall central tower that had defined the castle's silhouette for four centuries. After the blast, no one rebuilt it. The castle was used as a prison, most notably for Colonel John Lambert, a former Parliamentary general turned political inconvenience, who was held there from 1662 to 1670. The curtain wall around 1570 was later refaced and strengthened. The whole of Castle Cornet was listed as a Protected Monument on 26 March 1938. Today the noonday gun still fires from the battery at noon each day. The boom rolls across the harbor like a slow, low echo of all the cannonballs that once flew the other way.