Measuring just 6ft by 3ft and accessed by a trap less that a foot square, it is believed to have been used for spying on guests at Castle Fraser near Kemnay in Aberdeenshire.
Jewels and documents, such as maps, may also have been stowed in the “lug”, which was created some 400 years ago at the castle which has strong links to the Jacobites.
The Frasers of Inverallochy, who inherited Castle Fraser, were a branch of Clan Fraser of Lovat.
Charles Fraser of Inverallochy, whose father took ownership of Castle Fraser in the early 1700s, died on the field at Culloden after leading the Frasers of Lovat into battle.
Paula Swan, property manager at Castle Fraser, said the Laird’s Lug dates from between 1575 to 1618, when the 6th Laird of Fraser, Michael Fraser, embarked on a major renovation of the original medieval tower.
“We believe it may have been used to listen in on people in the Great Hall or for putting valuables,” she added.
The space now occupied by the Great Hall once formed part of the buttery, where servants would gather.
“Whether he was also building it to listen in on the servants, we will never know. All we really know is that it is there,” Ms Swan said.
Such is its size, staff at the castle had to build a special narrow ladder to access the Laird’s Lug during a recent cleaning exercise.
Castle Fraser was home to several notable Jacobites.
They include Charles, 4th Lord Fraser, who died accidentally in 1716 while attempting to escape from Government troops.
He died without issue and his stepson by his wife’s first marriage, William Fraser of Inverallochy (c.1672-c.1717), inherited Castle Fraser and its estates.
On William’s death the estates were inherited by Charles Fraser, affectionately termed ‘Auld Inverallochy’, who was also a staunch defender of the Stuart cause.
Several letters exist between Charles and Simon Fraser, 11th Lord Lovat, former chief of Clan Fraser of Lovat.
Lovat also corresponded with Charles’ son, also Charles, who was to die at Culloden.
The Aberdeenshire pile is not the only Scottish castle fitted with a Laird’s Lug.
At Edinburgh Castle, King James IV was able to spy on his subjects gathered in the Great Hall though a little barred window to the right of the fireplace.
Remarkable, it was bricked up in 1984 on security grounds ahead of a visit by Mikhail Gorbachev, then Soviet leader-in-waiting, following a request from the KGB.
Gorbachev’s trip to the castle was called off after he received news of the death of Konstantin Chernenko, his predecessor as General Secretary of the Communist Party of the Soviet Union.
The temporary brickwork has since been reversed.