The castle and nearby beach had to be decontaminated
A Highland castle attacked by Oliver Cromwell's New Model Army and later a site of experiments on radioactive liquid is at risk of collapse.
Engineers have carried out a structural survey of 16th Century Dounreay Castle following concerns over safety.
They said the ruins, which form part of the estate now managed by the Nuclear Decommissioning Authority, had badly eroded lintels and precarious masonry.
Historic Scotland has been informed of the latest survey results.
The government agency lists it as a scheduled monument and carried out its own inspection in February.
It is one of the few surviving examples of an L-shaped fortified tower house and its history has been researched by a member of staff at Dounreay Site Restoration Limited (DSRL).
William Sinclair of Dunbeath, descended from a younger brother of John Sinclair, third Earl of Caithness, built the castle in the 1560s.
It was badly damaged in 1651 by Cromwell's soldiers during their Scottish campaign.
However, it continued to be occupied up until 1863, but was without a roof and derelict by 1889.
Following the construction of Dounreay nuclear power complex in the 1950s, tests were run in the courtyard using a temporary holding tank and pipeline.
There was a leak and spillage and the site and nearby beach were contaminated.
A two-year project, which started in 1996, cleaned up the tower house and surrounding area.