Little development took place at the castle, which was not the family's principal residence and was mainly used as a holiday home in the summer months, but the interior was enhanced by Edward Douglas-Pennant's creation of a major collection of paintings.

In the 21st century, the National Trust's attempts to explore the links between their properties and colonialism and historic slavery have seen the castle feature in the ensuing culture wars.

[1] Despite losing his lands temporarily during the Glyndŵr Rising, Gwilym regained them by 1406 and began the construction of a fortified manor house and adjoining chapel at Penrhyn, which became his family's main home.

His son, Edward, rose to become Chief Justice of Jamaica and by the beginning of the late 18th century the family had accumulated sufficient funds to return to England, invest their profits in the development of their English and Welsh estates, and manage their West Indian properties as absentee landlords.

[11] The cost of the construction of this vast house is uncertain, and difficult to quantify as many of the materials came from the family's own forests and quarries and much of the labour from their industrial workforce.

[14] He noted the ingenious design of the bell pulls: "a pendulum is attached to each which continues to vibrate for ten minutes after the sound has ceased, to remind the sluggish of their duty.

"[14] He was even more impressed by the scale of Dawkins-Pennant's ambition; reflecting that castle building, which in the time of William the Conqueror could only be carried out by "mighty" kings, was by the early 19th century, "executed, as a plaything, — only with increased size, magnificence and expense, — by a simple country-gentleman, whose father very likely sold cheeses.

"[15] The elder of Dawkins-Pennant's two daughters, Juliana, married an aristocratic Grenadier Guardsman, Edward Gordon Douglas (1800–1886), who, on inheriting the estate in 1840, adopted the hyphenated surname of Douglas-Pennant.

Of Penrhyn, Hennesey wrote, "there is no building which illustrates so graphically the role which slave plantation profits played in the growth of British economic power.

"[29] In 2009 the Trust organised a symposium, Slavery and the British Country House, in conjunction with English Heritage and the University of the West of England, which was held at the London School of Economics.

The Common Sense Group of Conservative MPs challenged the Trust's priorities;[34] writing in a joint letter to The Daily Telegraph, "History must neither be sanitised nor rewritten to suit 'snowflake' preoccupations.

[39][d] Olivette Otele, Professor of Colonial History and the Memory of Slavery at the University of Bristol, explored the dominant narrative presented at Penrhyn after a visit in 2016.

"[42][43][e] Although it had already begun consideration of the links between its properties and the British colonial heritage, the murder of George Floyd and subsequent Black Lives Matter demonstrations, including the toppling of the statue of Edward Colston, led the Trust to acknowledge that these protests had given their efforts a greater impetus.

Writing at the time of the interim report's publication, Dr Katie Donnington wrote of the Trust's approach, "Is it scones and tea and a bit of Jane Austen-type fantasy?

[citation needed] In 2018, an art piece by Manon Steffan Ros, 12 Stori, was installed, which touched on the Pennants’ enslavement of African people.

According to the Trust's researcher Eleanor Harding, over the six months it was displayed, the Penrhyn staff and volunteer team felt insufficiently confident and knowledgeable to present this history sensitively and appropriately.

Harding of the National Trust explained that the aim of the exhibition was to place "the crucial role of empathy and emotion" at the centre of the audience’s engagement with the castle's history.

[51] Its origins lay in earlier instances of industrial unrest relating to the refusal of Lord Penrhyn and his agent to recognise the North Wales Quarrymen's Union.

[52] In 2018 local Plaid Cymru councillors accused the Trust of failing to fully recognise the contribution of slate workers to the castle's history.

[54][55] Penrhyn Castle houses one of the finest art collections in Wales, with works by Canaletto,[56] Richard Wilson,[57] Carl Haag,[58] Perino del Vaga,[59] and Bonifazio Veronese.

[67] Penrhyn is among the most admired of the numerous mock castles built in the United Kingdom in the 19th century;[68] Christopher Hussey called it, "the outstanding instance of Norman revival.

"[69] The castle is a picturesque composition that stretches over 450 ft (137 m) from a tall donjon, or keep, containing the family rooms, through the main block built around the earlier house, to the service wing and the stables.

[4] Haslam, Orbach and Voelcker, in their 2009 volume Gwynedd in the Pevsner Buildings of Wales series, describe it as "one of the most enormous houses in Britain" and note its "wholeheartedly Romanesque" style.

[g] The diarist Charles Greville recorded his impressions after a visit in 1841: "a vast pile of a building, and certainly very grand, but altogether, though there are some fine things and some good rooms in the house, the most gloomy place I ever saw, and I would not live there if they made me a present of the castle".

[73] Some modern critics have been similarly unimpressed; in his study The Architecture of Wales: From the First to the Twenty-First Centuries, John B. Hilling describes the castle as "nightmarishly oppressive, a most uninviting place to live".

The scale is immense, its seventy roofs cover an area of over an acre,[12] and its length, at 440 ft (134 m), which makes it impossible to be viewed in its entirety, disguises variations in the plan caused by the Pennants' desire to incorporate, rather than demolish, elements both of the original medieval house, and Wyatt's earlier castle.

"[12] It took over ten years to construct,[12] rises the full height of the house culminating in a lantern, its only illumination, and is built of a variety of grey stones decorated with "an orgy of fantastic carving".

[89] Haslam, Orbach and Voelcker think it Hopper's tour de force and see parallels with the contemporaneous approach in Gothic Literature, "antiquarian and anarchic, intended to play on the emotions as novels and poems were doing in words.

The lodge is condemned as "misapplied historicism"[94] while Hopper himself is censured as a model for the then-coming generation of Victorian architects, his career demonstrating how to "gain a whole world of rich commissions by eclectic dexterity, and still lose his own soul.

Its Cadw listing designation describes it as "one of the most important country houses in Wales; a superb example of the relatively short-lived Norman Revival of the early 19th century and generally regarded as the masterpiece of its architect, Thomas Hopper.