Wall paintings dating back hundreds of years have been found during a £3 million restoration of Lindisfarne Castle on Holy Island.
The discovery sheds new light on the history of the 16th Century castle which started as a fort above the North Sea, was renovated by Lutyens in the early 1900s and is now run by the National Trust.
As part of a programme of works, conservation experts found what originally seemed to be a series of butterfly motifs after carefully removing layers of paint and plaster in the kitchen and the west bedroom.
But further study showed the paintings were a stylised flower motif, likely to date to the mid to late 17th Century.
National Trust conservator John Wynn-Griffiths said: "This is such an exciting and rare find.
"We are always extremely careful when peeling back layers of history but we did not expect to find these paintings at all."
The decoration makes experts believe the building may have been more than merely a Northumberland base for soldiers guarding the volatile Borders.
Mr Wynn-Griffiths said the surviving paintings were not in the best condition, were professionally done and were found in different parts of the castle, indicating a more extensive decorative scheme.
Lindisfarne Castle opens to visitors on April 1 after the restoration and a section of the newly found paintings in the kitchen will be visible.
The building work will take some time to dry out, so the castle will be unfurnished for visitors.
Construction began in 1550 and it underwent significant changes over the next two centuries and was used as a fort until the mid-1800s.
It fell into disrepair by the start of the 20th Century until Country Life owner Edward Hudson commissioned architect Edwin Lutyens to develop it into a stately home.
It passed to the National Trust in 1944.