Positioned on cliffs overlooking the Bristol Channel, the site has been occupied since the Iron Age, and was by tradition the home of the Celtic chieftain Caradog.

Hearst undertook a "brutal"[2] expansion, including the incorporation of elements from other ancient structures such as the roofs of Bradenstoke Priory in Wiltshire and St Botolph's Church in Lincolnshire.

His approach to architectural reclamation was controversial and the destruction of Bradenstoke was opposed in a vigorous campaign organised by the Society for the Protection of Ancient Buildings.

According to tradition, the site of St Donat's was the place to which Caradog, the Celtic chieftain, returned after being released from imprisonment in Rome by the emperor Claudius.

[9] One of Edward's sons, Henry, was seized by pirates in the Bristol Channel while travelling from his Somerset estates to St Donat's, and was released only on payment of a large ransom.

[9] This event has subsequently been much embellished by, among others, Taliesin Williams in his account The Doom of Colyn Dolphyn: A Poem, with Notes Illustrative of Various Traditions of Glamorganshire,[10] which involves the eponymous Breton pirate and the witch Mallt-y-Nos.

[11][b] Henry Stradling's nautical misadventures continued; after acceding to the baronetcy, he died of a fever at Famagusta, returning from a pilgrimage to the Holy Land.

Sir Thomas Stradling (1495–1571) was imprisoned in the Tower of London in 1561,[13] following accusations of his having used the appearance of a "miraculous" cross in the trunk of an ash tree on the St Donat's estate to encourage support for the Catholic cause.

[14] His son, the scholar Edward Stradling (1528/9–1609) established a celebrated, and exceptionally large, library at St Donat's,[15] which was considered the finest in Wales of its time.

[8] The historian Graham Thomas records the Stradling tradition of educating their sons abroad, which led to the library holding extensive collections of foreign-language texts, particularly Italian works.

[19] He was also the patron of Siôn Dafydd Rhys and funded the production and publication of the latter's Cambrobrytannicae Cymraecaeve Linguae Institutiones et Rudimenta, the first Welsh language grammar to be published in Latin and thus widely accessible.

[28] Partial restoration was started by Dr John Whitlock Nicholl Carne, who claimed to be descended from the Stradlings, and bought the castle from the Tyrwhitt-Drake family in 1862.

[34] Godfrey Williams, Morgan's son, disliked St Donat's, by tradition on account of its being haunted[28] although this is disputed,[35] and in 1921, having first culled the herd of deer that his father had reintroduced to the park,[36] put the castle up for sale.

[43] Churchill's mention of "two charming wives" refers to Marion Davies, Hearst's long-time mistress and a constant presence at both San Simeon and St Donat's.

[46] He spent around £250,000 on repairs, reconstruction, refurbishment, and furnishings between 1925 and 1937,[47] renovating the castle with architectural trophies from across the United Kingdom and abroad;[48] at the peak of his buying, Hearst's expenditure reportedly accounted for a quarter of the world's entire art market.

"[49] The writer Clive Aslet described Hearst's passion for antiquities as "naked obsession... romance gave way to rape", and his mania for collecting was satirised in Orson Welles's 1941 film Citizen Kane.

[66] Much of the furniture, silver and works of art were disposed of in a series of sales conducted by Christie's and Sotheby's which began in 1937 and continued for some years,[67] with many items failing to achieve the prices Hearst had originally spent.

[5][70] In October 1939, the 2/5th Battalion of the British Army's Welch Regiment arrived at the castle; two years later, Auxiliary Territorial Service personnel were also stationed at the site.

[72][73] Hearst did not return after the war but continued to lend the castle to friends; Bob Hope, the comedian, stayed in May 1951 during his visit for a golf tournament at Porthcawl.

The idea for an international school arose from a meeting between the educationalist Kurt Hahn, who founded Schule Schloss Salem in Germany and Gordonstoun in Scotland, and Air Marshal Sir Lawrence Darvall, the commandant of the NATO Defense College.

[76] With Rear-Admiral Desmond Hoare, who would become the first headmaster, they persuaded Besse that the castle would make a suitable location for the first United World College, which opened in 1962 with fifty-six students.

[77][78] In an act of generosity, Hoare sold the patent for the boat's design to the Royal National Lifeboat Institution in 1973 for a notional £1;[77] the RNLI's cheque was not cashed and remains at the castle.

[79] From 1963 until 2013 the castle's seafront facilities hosted an RNLI lifeboat station,[80] which was staffed by students and faculty of the college, and was credited with saving ninety-eight lives along the South Wales coast during its period of operation.

[109] In an article in The Archaeological Journal, C. P. Spurgeon notes the design similarities between the tower's door jambs and those in the chancel of St Donat's church, indicating an earlier construction date than that of the work undertaken by Peter de Stradling.

[123] The historian Adrian Pettifer records St Donat's as the last inhabited castle in Wales to undergo major alteration, describing that Hearst "aggrandised it with plunder".

[124] The historian Anthony Emery, in the second of his three-volume history, Greater Medieval Houses of England and Wales, 1300–1500, describes the interiors created by Hearst and Allom at St Donat's as "spectacular...surpassing all other work there in size and richness".

[42] Hearst's breakfast room, off the banqueting hall, reuses another piece of the St Botolph's ceiling, as well as a fireplace from the prior's lodgings at Bradenstoke.

Alan Hall notes the similarity of the panelling to that in the Senior Common Room at Jesus College, Oxford, a foundation attended and supported by members of the Welsh gentry, including the Stradlings.

[59][l] In addition to the armour, Hearst assembled a considerable collection of art and antiquities at the castle; "must buy many things for St Donat's";[135] including a large number of 17th and 18th century English portraits, Classical Greek vases,[136] and tapestries.

[145] The Welsh poet Thomas Leyson, a friend of Sir Edward, composed a tribute in Latin, suggesting that the beauty of the gardens was sufficient to encourage visits from the sea-god Neptune and the water-nymph Thetis.