The earlier castle - Castle Girnigoe - was built by William Sinclair, 2nd Earl of Caithness, sometime between 1476 and 1496. We know it was finished before 1513, because that was the year William died at the Battle of Flodden along with much of the Scottish nobility. The castle may have been built on the foundations of an earlier fortalice. Robert Sinclair describes the structure as a five-storey L-plan crow-stepped gabled tower house perched on the rocky promontory, with a secret chamber hidden in the vaulted ceiling of the kitchen. In 1606 a second castle - Castle Sinclair - was added: a gatehouse, additional buildings, and a curtain wall connected to the older structure by a drawbridge over a ravine. The two together formed one of the most formidable seats in northern Scotland, and one of the strangest in design - a fortress that was really two fortresses, sharing a single dramatic site.

The castle's roll-call of inhabitants reads like a catalogue of misfortune and intrigue. In May 1573 the foud of Shetland - the sheriff of those northern islands - was brought to Girnigoe paralysed after a stroke. His name was Olave Sinclair. Four years later, in 1577, came the imprisonment and murder of John Sinclair, Master of Caithness, by his own father over suspicions of rebellion. In December 1594 the rebel Francis Stewart, 5th Earl of Bothwell - the nephew of Mary, Queen of Scots' notorious husband, and himself a man on the run from James VI - was at Girnigoe, sheltered by the Sinclairs. Later, George Sinclair of Keiss continued opposition to the senior Sinclair line and laid siege to the castle with firearms and artillery. He took it after what one chronicler called feeble resistance from the garrison. He, Sinclair of Broynach, Sinclair of Thura, and Mackay of Strathnaver were all declared rebels for their part - though political tides shifted and the declaration was quashed. Sinclair of Keiss eventually pursued his inheritance through the courts, with the help of the Duke of York, later King James II.

By the 19th century Castle Sinclair Girnigoe was a roofless ruin, slowly sliding into the sea. The Clan Sinclair Trust began archaeological research at the castle in 1998 and has been working ever since to preserve what remains. The castle has the distinction - unique in Scotland - of being listed by the World Monuments Fund as a site of international heritage importance worth saving. Conservation has stabilized walls, cleared rubble, and made the site safer to visit. The two-castles-in-one layout is still legible from the cliff path: the older inner tower-house on its rock, the newer outer gatehouse and curtain wall, the ravine between them spanned in the imagination if not in stone. From the seaward edge, the drop is sheer. The Battle of Champions was fought just west of here, around 1478, and Castle of Mey lies an hour up the road. This stretch of Caithness coast carries more medieval blood per square mile than almost anywhere in Scotland.