The new research suggests construction began around 1369 or 1370, immediately after war with France resumed at the end of the truce that had followed the Treaty of Bretigny. Sir Renaud de Carteret, Seigneur of St. Ouen, owned the headland. His family had been wrecked once already by French raiders, the manor at St Ouen destroyed during the invasions of 1338 and 1339, the family exiled to Normandy. When war returned, Renaud built fast and built where nobody could land easily, on a cliff-edge promontory that nature had already nearly fortified. The castle was probably not finished when the French came back. In 1373 the Duc de Bourbon and Bertrand du Guesclin, Constable of France, descended on the Channel Islands looking for booty and ransom. They took Grosnez Castle quickly, slighted it according to the medieval rule (destroy what you cannot occupy so the enemy cannot use it), and forced Jersey to pay tribute until the end of 1375.

After the slighting, the documents fall silent. For five hundred years nobody is sure what happened on the headland. The most likely answer is mundane: the stones walked. Building stone was scarce and expensive in medieval Jersey, and a ruined castle full of already-shaped granite would have been irresistible to anyone with permission and a cart. In 1483 Sir Philippe de Carteret, Renaud's descendant, received royal permission to crenellate his manor house at St Ouen. He was investing heavily in restoring buildings his guardians had let go to ruin, supposedly to the point where trees grew in the hall. By the nineteenth century, when the Societe Jersiaise sent in archaeologists in the 1880s and 1890s, the castle had been so thoroughly robbed of stone that only the top of the gatehouse stuck above the rubble. The first task of the excavation was clearing the debris off the foundations. They are working with a stripped skeleton.

What does remain is dramatic. The gatehouse arch still stands, framed against the Atlantic horizon like a doorway into nothing. Walking up to it from the gorse-covered moor, you pass through that arch into thin air and a view that runs unbroken to Sark, Guernsey, and on clear days the Casquets light off Alderney. Below the cliffs, peregrine falcons hunt along the wind. Kestrels hover above the heather. Red-billed choughs, reintroduced to Jersey, work the cliff faces with their distinctive call. The bedrock here is coarse-grained granite of the St Mary's type, the same stone the castle was built from, the same stone the islanders use to mark their boundary walls a thousand years later. Common heather, bell heather, cross-leaved heath, and Western gorse give the headland its colors: pink and yellow in summer, dark green and rust in winter.

Modern Jersey has done what modern islands do with their ruins. Grosnez Castle is on the reverse of the Jersey fifty-pence coin and on a dozen postage stamps. The site is unmanned and free, open to anyone willing to walk out across the moor from the car park at the end of the road. Because of how remote it is, with no streetlights and almost no nearby houses, the headland is one of the best places in the Channel Islands to look at the night sky. The Milky Way runs from horizon to horizon. Sometimes, when conditions align, the aurora borealis flickers along the northern edge of the sea. The castle the chronicles forgot is the kind of place stargazers find each other in the dark, the gatehouse arch a black silhouette against a sky too crowded with stars to count.