The final phase of improvements is now underway at Lincoln Castle.
Work has begun on the refurbishment and restoration of the Georgian and Victorian prison buildings and the construction of an underground vault to display Magna Carta and the Charter of the Forest.
The work in the prisons involves the creation of a cinema screen and auditorium to showcase the story of Magna Carta
The historic east-wing staircase will also be reinstated, with improved access to the new exhibition area, new visitor facilities, including a ticket office, gift shop, café and toilets, and improved disabled access.
The improvements will be carried out by Robert Woodhead Ltd, who recently created the new Heritage Skills Centre, the first new building in the castle grounds for 150 years.
The castle will remain open to the public throughout. Work on the project is due to be completed in time to celebrate the 800th anniversary of Magna Carta in 2015.
Councillor Nick Worth, Executive Member for Heritage, said: “This is a real milestone moment for the project.
“The castle is the only place in the world where Magna Carta and the Charter of the Forest can be seen together, and this phase of the improvements will give them the home they deserve.
“This will make the castle an attraction of international importance, bringing in many more visitors from near and far. And that’s good news for the local economy.”
Simon Butler from Woodhead Heritage added: “We are very pleased and excited to have won the fourth and latest project at Lincoln Castle as this type of project is very rare.
“The prison buildings are Grade II listed and will require careful conservation; at the same time, we want to ensure the final buildings offer a world-class visitor experience.”
The project has been estimated at £19.9 million, with £12 million from the Heritage Lottery Fund, £5 million from the County Council and £1.1 million of European funding.
The remaining £1.6 million will be raised through further fundraising through the Historic Lincoln Trust.