A former stronghold of the Sinclairs, the ruins of Castle Sinclair Girnigoe crowns a small rocky peninsula a half-mile (1 km) west of Noss Head, at the southern end of Sinclair's Bay on the east coast of Caithness in Highland Council Area and 2½ miles (4 km) north northeast of Wick. This fortress was once thought to comprise two quite separate castles located next to one-another, with Girnigoe Castle said to date from the later 15th century and Castle Sinclair from 1607. However after detailed investigations, archaeologists reported in 2003 that these were actually parts of the same structure and that it is a century older than previously suspected, most likely built by the Norse Earls of Caithness. The western part of the castle ('Castle Sinclair') appears to have been a gate-house and little remains other than a tall chimney stack. Separated by dry moat, the eastern ('Castle Girnigoe') section is the better preserved, forming a U-plan with a main block reaching to five storeys and two projecting wings which rising a storey higher. The kitchen was in the vaulted basement, from which a small stair climbs to the Great Hall above. This was the principal entertaining space in the castle, and features an ornate oriel window. Evidence suggests a tower-house was built here in the late 14th century, known as Castle Girnigoe. It was not until 1606 that George Sinclair, 5th Earl of Caithness (1566 - 1643), decided the whole should be known as Castle Sinclair and proposed an Act of Parliament to effect this change. In 1623, the same Earl was proclaimed a rebel and fled to Orkney. King James VI commissioned Sir Robert Gordon (1580 - 1656) to give chase and Gordon took possession of the castle.
Castle Sinclair Girnigoe was occupied by Oliver Cromwell's forces in the 1650s but, in 1679, it was rendered uninhabitable by George Sinclair of Keiss to prevent it being taken by Campbell of Glenorchy, who claimed the property and title of Earl of Caithness as settlement of the debts of another George Sinclair, the 6th Earl (d.1676).
The castle is a scheduled ancient monument and is now owned by the Clan Sinclair Trust, a charity formed in 1999 under the patronage of HRH Prince Charles, Duke of Rothesay and supported by Malcolm Sinclair, 20th Earl of Caithness (b.1948), and John Sinclair, 3rd Viscount Thurso (b.1953). Because of the confusion between its two names, the Trustees decided to refer to the structure as Castle Sinclair Girnigoe in 2002. In the same year, the castle was also listed in the Watch List of the World Monuments Fund, which catalogues the hundred most endangered sites in the World, and is the only castle in Scotland to be so listed.