Three separate sections of wall have been unearthed in Kirkwall town centre.
Archaeologists said the remains were of Kirkwall Castle, which was built without royal consent in the late 14th Century by Earl Henry Sinclair while Orkney was still ruled by Scandinavian kings.
It is said to have been one of the strongest castles in the realm. When under fire in the early 17th Century it is said canon balls “split like wooden golf balls against the walls.”
A team from ORCA Archaeology made the discovery as Orkney Islands Council push on with infrastructure projects in the centre of Kirkwall.
Pete Higgins, Senior Project Manager ORCA Archaeology commented, “ This is an area of the city that we know was the site of the castle and it is exciting to see the remains of the possible curtain wall and part of the fourteenth-century Kirkwall Castle in situ..”
The whole site will be recorded, added to the historical archive and covered over again so that the infrastructure works can progress without delay.
King James VI issued an order to dismantle Kirkwall Castle in 1615 so that it could not be used again as a centre of rebellion.
This process of destruction was completed in 1865 when the remaining structure was demolished to make way for Castle Street.
No visible signs of the once-immense fortification can be seen above ground.
To date, three walls in total have been uncovered during the works. One substantial wall set back from the road junction is built using immense stone blocks and lime mortar.
Previous building works in the 1980s revealed massive stone walls close to the present site which most likely were the foundations of the castle.