Standing on a rocky promontory in the sea, Girnigoe Castle and Castle Sinclair were two separate fortresses, although they stood very close together, not much remains of Castle Sinclair.
Girnigoe Castle is an altered keep, and consists of a main block and two projecting wings, creating a u-plan building. It had a walled courtyard, within which Castle Sinclair was built, which had a gatehouse with a drawbridge to span a deep ditch
cutting off the promontory.
The main block of Girnigoe is of five storeys but, due to the uneven level of the ground the main block basement is a storey lower than the wings. The entrance was in the stair-wing. The vaulted basement contained the kitchen, with a small stair
up to the hall above. The hall, on the first floor, has been a fine chamber, and has a decorated oriel window.
In 1472 the Sinclair Earls of Orkney were forced to resign the Earldom and were given Caithness as compensation. The second Earl, William Sinclair, built a castle here but died at the Battle of Flodden in 1513. In 1571 the fourth Earl had John,
Master of Caithness, his son and heir, imprisoned in the Dungeons for seven years. The master was fed on salted beef, and denied water so that he died mad with thirst.
The fifth Earl built Castle Sinclair, but feuded with the Earl of Sutherland, and put down a rebellion in Orkney in 1615. Cromwell had the castle garrisoned in the 1650s. The sixth Earl had huge debts to Campbell of Glenorchy, and the Earldom was
claimed by the Campbells. In 1679 the castle was besieged and captured by Sinclair of Keiss, who also seized the Earldom. The Sinclairs were brought to battle at Altimarlech, where they were slaughtered in such numbers that the Campbells
reportedly could cross the river without getting their feet wet. However the Sinclairs recovered the Earldom in 1681. The castle was damaged during the attack and not re-used. The property passed to the Dunbars of Hempriggs, but was sold back
to the Sinclairs about 1950 and is now held in trust.