A flag is usually flown when the Duke and Duchess of Northumberland are in residence. But on Friday, instead of the Percy lion flying proudly in the breeze, a flag was emblazoned with the words: Sarah Darling 50.
The flag was raised to celebrate the 50th birthday of Sarah Darling, of Lucker, who is personal assistant to the Duke and Duchess.
Sarah has worked at Alnwick Castle for 23 years and her colleagues, together with the Duchess, arranged the special surprise.
A flag has never before been raised at the castle for a person who is unrelated to the Duke and Duchess.
Sarah said: "I thought at first, when I glanced up, that it was a strange day for the flag to be flying. Then the words were pointed out to me and I got a huge shock.
"It was so embarrassing but I feel hugely honoured that this has been done for me. It's made today a very special occasion."
Members of Sarah's family were also shocked to see the flag. "My niece saw it because she goes to Alnwick Duchess's High School. She couldn't believe it."
There were also huge birthday banners on display for Sarah at the gatehouse entrance to the castle.
Sarah's birthday was on Monday. She celebrated with family and friends over the weekend before going on holiday to Antigua.