It is so fortunate that Julian Fellowes, a friend of the Highclere owners Lord and Lady Carnavan, saw the potential in building an entire fictional world around this spendid home. He had done it once before with the Ardverikie Estate in Scotland, with the hit series Monarch of the Glen. In fact Monarch of the Glen, the story of a Scottish family estate trying to avoid bankruptcy by making the house a tourist attraction, actually tells the story of Highclere Castle much better than Downton Abbey does. Downton Abbey may be representative of Highclere's past, but Monarch of the Glen is a much better representation of it's present and future.
Read more about it here in this 2009 story in the Daily Mail.