The dominating stone façade of Penrhyn Castle hides more than just its internal red-brick construction. Built in the early 19th century, its formidable architecture, opulent interiors and fine art collection lean on a long history of sugar and slate fortunes, social unrest and the longest-running industrial dispute in British history. Penrhyn Castle was built using profits made by a family that used "hundreds of enslaved people".

Penrhyn is one of the most admired of the numerous mock castles built in the United Kingdom in the 19th century.

In the early 20th century, relations between Penrhyn and local communities began to deteriorate. Exploitation of the workers in the quarry led to the longest running industrial dispute in Britain’s history. Beginning in 1900 it centered on Union rights, pay and working conditions. The Great Strike was a bitter battle between Lord Penrhyn and the quarry workers, the effects of which are still felt today.

In 1949, after the death of the fourth Lord Penrhyn, the land and title separated. The title went to Frank Douglas Pennant, who became fifth Lord Penrhyn, and the land went to the fourth Lord’s niece, Lady Janet Harper.

Only two years later, Penrhyn Castle, along with the Ysbyty Ifan and Carneddau estates, came under the care of the National Trust.