Emergency stabilisation work is being carried out to a 13th century keep once occupied by the Jacobites.
The £37,000 project at Skelbo Castle, near Dornoch, and the nearby Old Skelbo House, will prevent the scheduled monument falling into further disrepair.
The bulk of the scheme will concentrate on stabilising masonry and walls of the keep and the mansion house, while the remaining parts of the roof structure of the Old Skelbo House will be dismantled.
Historic Scotland has commissioned the work, despite it being unable to contact the foreign owner of the property.
A spokeswoman said: “We have been concerned for several years about the declining condition of the castle. In addition, there has been considerable local concern.
“The castle is privately owned and we have been trying to communicate with the owner, who is believed to live abroad, about the castle.
“However, ongoing attempts have so far proved unsuccessful, despite an offer to fully grant aid emergency works. We commissioned a condition survey in 2008 which highlighted the areas of the castle which were at most risk of collapse.
“In light of this and our inability to communicate with the owner, Historic Scotland has now commissioned emergency works to ensure the continued survival of this nationally important monument, under the provisions of the 1979 Ancient Monument and Archaeological Areas Act.”
The contractor, Thurso-based John Munro Joinery and Building Services, is now on site and scaffolding has been erected around the keep and the mansion house.
The spokeswoman added: “Since the work began it has become clear that the castle has deteriorated during the winter and that a section of the 13th century curtain wall was at imminent risk of collapse.
“The emergency works will stabilise this and other sections of the castle most at risk and will give all interested parties time to decide how best to secure the castle.”
The keep overlooking Loch Fleet is thought to have been occupied by the Jacobites in 1746 but was ruinous by 1769.