A major conservation project has started at English Heritage’s Carlisle Castle this week.

Built in 1092 by William II, Carlisle is the most besieged castle in England.

In 1315 Robert the Bruce tried to take it and in 1568 Mary, Queen of Scots, was held captive in one of its towers.

It featured prominently in the English Civil War and Jacobite troops under Bonnie Prince Charlie fought over it in the 18th Century.

English Heritage says the project aims to address issues with water damage within the castle keep, both at ground and roof level.

According to a spokesperson from English Heritage, specialist workers will be working to repair parts of the roof and some masonry, ensuring that drainage at ground level is improved and adding in specialist heating to the roof to stop the water seeping in.

Within the keep, workers will repair stonework and ensure that the Prisoner carvings are safe.

The stonework etchings, which include images of mermaids, horses and stags, could date as far back at the 15th century and were probably made by prison guards on duty.

A special technique called ‘photogrammetry’ has been used to three dimensionally capture data, which will soon be analysed to discover what these strange and delicate carvings mean.

Carvings by prisoners are not unusual and can also be seen at places like the Tower of London, Dover Castle and Windsor Castle, but the style and date of the carvings at Carlisle Castle make them unique.

Juliet Fellow-Smith, English Heritage Property Manager at Carlisle Castle said: “This is another significant day in the history of this much-loved castle.

"It is a poignant time given that the keep was built in stone 900 years ago this year.

"We are so pleased to see the work finally starting and ensuring that both the historic fabric and wall art are protected for years to come.”

The project should be completed in the summer.