Consulting engineers Cruden Associates are carrying out a structural survey of the 16th century Dounreay Castle.
The castle, which forms part of the estate inherited by the Nuclear Decommissioning Authority, is in a ruinous condition.
Dounreay Site Restoration Ltd recently informed Historic Scotland, the Government body with responsibility for building heritage, of further deterioration in its condition.
The lean of the north wall has increased considerably, lintels have eroded and weakened and some sections of masonry are now unsupported, creating a risk of sudden collapse. Restrictions on access have been imposed on safety grounds.
Historic Scotland inspected the building in February. A.F. Cruden carried out a structural survey today for DSRL.
The results of the survey will be discussed with the NDA and Historic Scotland.