About
Castle Cornet has guarded the mouth of St Peter Port harbour for more than eight hundred years, standing on a tidal islet that was joined to the town by a breakwater only in the nineteenth century. Building began in the early thirteenth century, when King John's loss of mainland Normandy in 1204 left the Channel Islands as an exposed English frontier facing a hostile French coast, and Edward I issued the formal charter for its construction in 1275. For most of its working life the castle was both the island's principal fortress and the official residence of the Governor of Guernsey, a role it lost abruptly in 1672 when lightning struck the magazine and the resulting explosion destroyed the medieval keep, killed the Governor's wife and mother, and left the high castle in ruins.
Its most celebrated episode came during the Civil War. Guernsey declared for Parliament, but the Royalist Governor, Sir Peter Osborne, held Castle Cornet for the King from 1643, bombarding his own island's capital from the harbour mouth and being supplied by sea from Jersey and France. The garrison held out for eight years and did not surrender until December 1651, making Castle Cornet the very last Royalist stronghold in the British Isles to fall — a distinction it shares with nowhere else.
The Crown gave the castle to the people of Guernsey in 1947, and it now works as a heritage site containing four separate museums: the Story of Castle Cornet, the Royal Guernsey Militia Museum, the Royal Guernsey Light Infantry Museum, and the 201 Squadron Royal Air Force Museum. Five period gardens are laid out within the walls, and German fortifications from the 1940-45 occupation survive alongside the medieval and Tudor stonework. A noon-day gun is fired from the ramparts each day of the season by gunners in nineteenth-century Guernsey Militia uniform, a ceremony that draws a crowd on the walls above the harbour.
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Plan Your Visit
⏱️ Recommended Visit Duration
💡 Visitor Tips
- Check the season before you travel — in 2026 the castle opens only from 25 March to 6 September and is shut entirely outside those dates.
- Arrive by about 11:30 if you want a good position on the ramparts for the noon-day gun; it is fired every day of the season.
- Warn children and anyone noise-sensitive about the gun in advance — the report is genuinely loud and echoes off the harbour walls.
- Wear proper footwear: the climb through the castle is on cobbles, ramps and steps, and it is steep in places.
- Buy the £24 Discovery Pass if you also plan to see Guernsey Museum at Candie, Fort Grey or the German Naval Signals Headquarters — it costs twice a single castle admission and lasts a year.
- Allow at least two hours; four separate museums plus the gardens and the fortifications take longer than most visitors expect.
- The castle is exposed on all sides at the harbour mouth, so take a windproof layer even on a warm day.
- Walk out along the breakwater rather than looking for parking on the emplacement — there is none at the castle itself.
Highlights
The Last Royalist Stronghold, 1643-1651
Guernsey declared for Parliament in the Civil War, but the Royalist Governor Sir Peter Osborne held Castle Cornet for the King from 1643, bombarding St Peter Port from the harbour mouth while being resupplied by sea from Jersey and from France. The garrison endured eight years of siege in a castle cut off from its own island's population, and did not surrender until December 1651 — making Castle Cornet the last Royalist position in the British Isles to fall, months after the war was otherwise decided.
The castle's interpretation gives this episode considerable space, and it remains the single fact most visitors take away.
The Noon-Day Gun
A cannon is fired from the castle ramparts every day of the visiting season, at noon, by gunners in the nineteenth-century uniform of the Royal Guernsey Militia. The ceremony is brief but genuinely loud, and it draws a crowd both inside the walls and along the harbour front below, where the report echoes off the town.
It is worth timing a visit to arrive well before twelve in order to get a position on the upper works, and worth warning children and anyone sensitive to sudden noise beforehand.
The gun crew give a short explanation of the drill before firing.
The 1672 Lightning Strike
On a December night in 1672 lightning struck the castle's powder magazine and the explosion tore apart the medieval keep at the summit of the rock. The Governor's wife and mother were among those killed; the Governor himself survived. The keep was never rebuilt, and the loss ended Castle Cornet's role as the official residence of the Governor of Guernsey, which moved into the town.
The consequence is visible everywhere on a modern visit: the highest part of the castle is an open platform of foundations and later gun positions rather than a standing medieval tower, and the site's whole silhouette is a product of that single night.
The Royal Guernsey Militia Museum
The Royal Guernsey Militia was among the oldest militia forces in the British Isles, raised from the island's own population and maintained for centuries as Guernsey's standing defence against French raids and invasion. The museum covers its long history through uniforms, weapons, colours, medals and documents, and sets the force in the constitutional context that makes it unusual: Guernsey is a Crown Dependency with its own legislature, and its militia was an island institution rather than a British Army regiment.
The displays trace the force from its medieval origins to its dissolution, and explain how militia service shaped ordinary Guernsey life.
Collections
Castle Cornet is both a fortification of the thirteenth to twentieth centuries and the home of four distinct museums. The Story of Castle Cornet interprets the site itself, from its construction after the loss of Normandy in 1204 through the eight-year Royalist siege of 1643-51, the destruction of the keep by lightning in 1672, and the German occupation of 1940-45.
The Royal Guernsey Militia Museum covers one of the oldest militia forces in the British Isles; the Royal Guernsey Light Infantry Museum records the island battalion raised in 1916 and shattered at Cambrai in 1917; and the 201 Squadron Royal Air Force Museum documents the maritime patrol squadron affiliated to Guernsey.
Five period gardens are laid out within the walls, and a noon-day gun is fired daily in season.
The Story of Castle Cornet
Eight centuries of the castle's history, from its thirteenth-century construction to the Civil War siege, the 1672 explosion and the German occupation.
Royal Guernsey Militia Museum
Uniforms, weapons, colours and documents of one of the oldest militia forces in the British Isles.
Royal Guernsey Light Infantry Museum
The island battalion raised in 1916, its service on the Western Front and its losses at Cambrai in November 1917.
201 Squadron Royal Air Force Museum
The history of the RAF maritime patrol squadron affiliated to Guernsey, from the Royal Naval Air Service to the Nimrod era.
Themes
Time Periods
Current Exhibitions
No current exhibitions. Please check the museum website for updates.