Hay Castle, one of the great medieval defence structures on the border of Wales and England will open its doors to the public this weekend.
Built in the late 12th century, the castle has been privately owned for 800 years and is rarely open to the public.
Visitors will get a chance to explore the building and hear the Trust's plans for the future.
The Prince of Wales and Duchess of Cornwall will be in Hay on Wye today visiting local businesses and perusing the wares at the town's Thursday market.
Among the stops on the Royal visit will be family-run country store FW Golesworthy & Sons, the town's largest bookshop Richard Booth's Bookshop, the vacant Cheese Market and Hay Castle.
Prince Charles and Camilla will then tour the Hay Festival site, before joining authors at a School Library Association reception.
Prince Charles, in his role as patron of The Prince's Countryside Fund, will meet local farmers during the visit, while the Duchess will be introduced to schoolchildren at the launch of the Summer Reading Challenge.
The Royal couple will finish their visit by attending the opening night of the Welsh National Opera's new production of Wagner's 'Lohengrin' at the Wales Millennium Centre in Cardiff.