Image copyright Chris Jackson Image caption Prince Charles and the Duchess of Cornwall attend the reopening of Hillsborough Castle

The Prince of Wales and Duchess of Cornwall have officially reopened Hillsborough Castle after its extensive five-year makeover which cost £20m.

The prince said that for him, Hillsborough had always been a "special place" which reflects the "complex history of this island".

The County Down site is the Queen's official residence in Northern Ireland and home to the secretary of state.

The castle and its new visitors' centre will open to the public on 18 April.

The Georgian mansion, dating back to the 1770s, is set within 100 acres of grounds and is best known for hosting royal garden parties and Northern Ireland peace talks.

Image copyright Chris Jackson Image caption The castle is hoping to attract thousands of new visitors every year

In 2014, the estate's management was taken over by the independent charity, Historic Royal Palaces, which led a major refurbishment programme in a bid to make it more accessible to the public.

This involved restoring state rooms, redeveloping a four-acre walled garden and building a new visitor car park off the A1.

Royal guidance

In a welcome speech, Historic Royal Palaces chairman Rupert Gavin told his royal guests the official opening was a "major milestone in the long history of Hillsborough Castle".

He said the refurbishment was a "Herculean task" involving hundreds of people.

Image caption Rupert Gavin said Prince Charles took a personal interest in the restoration

"It will be a place that the whole community can engage with and hopefully Hillsborough will be a massive resource for all parts of and all sections of Northern Ireland," Mr Gavin added.

The chairman also revealed that the charity had enjoyed the support, encouragement and "occasionally detailed notes" from Prince Charles during the restoration.

'Extraordinary transformation'

The prince said more than 700 contractors and craftspeople had been involved in the renovation and he paid tribute to their efforts.

"I must congratulate each and every one of you for the part you have played in this historic project."

He added that the "investment in the project has been vast and the transformation quite extraordinary".

Image copyright Getty Images Image caption The Royal couple's visit coincided with their 14th wedding anniversary

The prince claimed that, previously, Hillsborough Castle was "one of those places that many had heard of, yet few had ever visited".

"The task of the dedicated team at Hillsborough and at Historic Royal Palaces was to open up the castle and to tell its stories more widely," he explained.

"As we look to the future therefore, I hope Hillsborough Castle and gardens can now well and truly be placed on the map and serve as both a destination and indeed an inspiration for all on the island of Ireland to enjoy."

Young people from his charity, the Prince's Trust, have been involved in creating a new pavilion in the walled garden, which is still under development.

Image copyright Chris Jackson Image caption Prince Charles, who is known to be a keen gardener, toured the refurbished walled garden

The Royal couple, who are celebrating their 14th wedding anniversary, were greeted by children from Downshire Primary School and other invited guests.

They unveiled a plaque and were given a tour of the house and new visitor facilities.

The project received millions of pounds of support from the Heritage Lottery Fund (HLF).

Historic Royal Palaces also looks after other royal sites such as Kensington Palace, Hampton Court Palace and the Tower of London.