An 18th century Gothic castle in Twickenham will open its doors to the public after a £9m restoration project.

Strawberry Hill House, in Waldegrave Road, will officially open to visitors on Saturday after a two-year long restoration on the first phase of the building.

The Grade I listed building was built by the writer and son of Britain's first Prime Minister, Horace Walpole, between 1747 and 1792 as a Gothic fantasy. It is one of Britain's finest examples of Georgian Gothic Revival architecture.

Having fallen into a state of extreme disrepair, the house had been on English Heritage's at risk register since 1991.

It was also listed by the World Monuments Fund as one of the world's most endangered heritage sites in 2004, a move which was the catalyst in starting a campaign for its repair.

One of the key organisations behind the project is the Strawberry Hill Trust, which formed in 2002.

The group took over a 120 year lease from the Catholic Education Council which purchased the estate in 1923, to begin work on its restoration.

The project has been part funded by a £4.9m grant from the Heritage Lottery Fund.

With the rest of the funding coming from donations by English Heritage, World Monuments Fund Britain, the Architectural Heritage Fund, as well as numerous charitable trusts, local societies and individual patrons.

Carole Patel, a member of the Strawberry Hill Trust, said: “We wanted to get it looking exactly how Walpole had built it, or at least not far off.

“The house was just originally just two old cottages, but over the years Walpole extended it. He wanted to take people on a journey as they wandered through the house.

“Downstairs he kept the décor very basic. It almost feels like you are in a monastery with the grey stone walls, cloisters and narrow arched windows, but upstairs the rooms open up and are bursting with colour and detail.”

The house will also have an education centre for schools and colleges to use, a small museum documenting the history of the house and some artefacts, as well as a cafe, and gift shop.

Work will begin on the second phase of the building in January 2011.

It will involve restoring the gardens to how they originally were and returning some of Walpole's original furniture, paintings and other artefacts to the house - some of which were recently on display at the Victoria and Albert museum.

Carole Souter, chief executive of the Heritage Lottery Fund, added: “Back in the 1780s, Horace Walpole created a little bit of heritage magic in Twickenham.

"Fast forward several centuries and Strawberry Hill is once more resplendent in its Gothic glory and ready to throw open its doors open again to visitors.

“The Heritage Lottery Fund's support has helped safeguard the villa's future, ensuring people of all ages will enjoy its wonderful architecture as well as learn about its central role in our country's fascinating political history.”