Auckland Castle was saved last year by a Â£15m donation from businessman Jonathan Ruffer, after Church Commissioners tried to sell it.
It was then handed over to a new charitable trust, The Auckland Castle Trust, who have redeveloped the site into a heritage tourist attraction.
The revamp coincides with Durham County Council's redevelopment of the town.
Paintings by Spanish artist Francisco Zurbaran, which hung in the castle for 250 years, were also included in the deal.
The former home of the Bishops of Durham now has guided tours, a children's trail for younger visitors, a new tearoom and an exhibition of religious artwork.
Chief executive Rowena Hackwood, said: "We can't wait to welcome visitors from Country Durham and the wider region and to showcase some fantastic new features which kick-start our major development plans.
"Auckland Castle offers something for everyone and is not just a destination for art aficionados and lovers of history, culture and architecture."
Over the next five years, work will be carried out to give public access to new areas of the site and its grounds and the 17th Century walled garden and the 14th Century Scotland Wing will be restored.
A County Durham castle which is home to a collection of 17th Century religious paintings has reopened.