Archaeologists have hailed a discovery at a castle near Aberdeen which has lain hidden for at least 150 years.

Work to restore the medieval tower at Drum Castle, 12 miles west of Aberdeen on Royal Deeside, has revealed a secret chamber where a Jacobite hero of the Battle of Culloden hid out for three years, and a second chamber - the guarderobe (toilet), including the remains of the original toilet seat

Alexander Irvine, the 17th laird of Drum, fought for Bonnie Prince Charlie and escaped after Culloden. He sheltered in the secret room at Drum and was saved from capture by the Redcoats by his sister, Mary, who misdirected them.

The archaeological investigations are being conducted by Dr Jonathan Clark from FAS Heritage.

Dr Clark said: "We knew that there were hidden passages because there were window openings at first floor level, but we couldn't see from the inside of the tower where the windows were because they are hidden by the bookshelves of the nineteenth century library.

"So we set out to unblock two window openings on the west face of the tower to establish the form and condition of these interior spaces. Before we unblocked the windows we wondered if the passages had been filled up with rubble at some point in the history of the evolution of the ancient tower and that there would be nothing to see.

"We were surprised that when we carefully unblocked the windows and peered in, and through the dim light of a torch and the mists of dust and trapped for centuries, to find a perfectly preserved medieval chamber, complete with the remains of the guarderobe (toilet) including the remains of the original toilet seat and the original entrance doorway for the medieval hall."

He added: "This adds greatly to our knowledge of how the interior of the Tower of Drum was used in the medieval period. In due course it should contribute to a greater knowledge of how fourteenth century towers were used in their heyday.

"And, as work continued this morning, we made another exciting discovery - a second chamber which legend says is where Mary Irvine hid her brother for three years after defeat in the Battle of Culloden. This is a huge discovery for Drum.

"We will now be carefully photographing and measuring what we have discovered so that we can add it to the plans that we have been preparing on the Tower of Drum as part of the bigger project of conservation and archaeological investigation on this important castle."

The £700,000 project will repair structural problems on the tower and remove and replace its mortar. The work is generously funded by a grant awarded by the National Trust for Scotland Foundation USA made possible by the generous support of an anonymous donor and Historic Scotland.

Throughout the works to the tower, the castle, gardens and grounds remain open for visitors.