The origins of Fonmon castle are poorly documented and most of its early history has been discovered through studying its architecture. A popular myth exists that the castle was built by Oliver St John of Fonmon, one of the Twelve Knights of Glamorgan who effected the Norman conquest of Glamorgan. It has since been shown that this was a legendary tale given credence by a "historic" recount in the 1560s by Sir Edward Stradling. There is speculation that a defensive fortification made of timber was built on the site of Fonmon Castle soon after the Norman invasion of Wales, with stonework added around 1200. The thick walls to the left of the entrance is easily identifiable as 12th century in design, and shows the existence of a rectangular keep. This keep was relatively small in size, approximately 8m x 13m. In the 13th century a curtain wall was added to the east side of the keep leading towards a steep scarp. This was followed by a much larger L-shaped build to the south with a south-east tower added at the angle. The east wall along the ravine edge has the castle's thickest walls, which is a curiosity as this would have been the most difficult side to attack. It is therefore believed that some other defensive structure may have existed to protect the more vulnerable south and west approaches. These early constructions are thought to have been undertaken by the St. John family, who were associated with the castle during the Middle Ages and continued as owners until 1656. It was not until the 16th century that the next major addition was made, a short north wing built over a barrel-vaulted semi-basement. The castle survived being damage during the English Civil War, with the St. John family supporting the Parliamentarians, but shortly thereafter they fell into financial difficulties and the castle was sold to Colonel Philip Jones. It is believed that Jones then improved the rooms on the east range and added a double depth wing to the north side. Upon Jones' death the castle was passed down to his son Oliver who owned the castle from 1678 to 1685. It was Oliver's great-grandson, Robert Jones III, who was to make the next major redevelopment to the castle. He married Jane Seys, heiress to the Seys of Boverton and in 1762 they began the castle improvements, employing Thomas Paty of Bristol. The firm added render and made additions the battlements to give it more of an appearance of a castle. The estate went into a period of decline in the 19th century and little work was done to the castle except for the addition of the entrance porch and the extension to the south wing in the period between 1840 and 1878. The castle passed by marriage to Sir Seymour Boothby of the Boothby baronets in 1917 and his grandson still lives there. The castle is open to the public on Tuesday and Wednesday afternoons from April to September; access to the garden and grounds is free. It is also a wedding and events venue, hosting the Vale of Glamorgan Agricultural Show since 1998 and the St David's Polo Club. The castle and estate was placed on the market for sale during April 2018. The asking price for the estate as a whole is £11.5 million.
Architecture
Exterior
Fonmon Castle is situated in extensive gardens and is constructed of local sourced stone, primarily limestone and blue lias rubble. There is one arched internal doorway which appears to be Sutton stone; but are covered in grey render which disguises much of the stonework. The roofs are of mixed slates with lead gutters and dressings. The main building is of two and three storeys and castellated almost throughout. Apart from the south east corner tower, which is slightly higher, the walls are largely uniform in height. In the grounds, to the south west of the house, there is an 18th-century stable which incorporates a late medieval barn. The south and east walls of the stable are castellated to impress those approaching from the south. The stable has a fine polygonal stone chimney, which is a rare surviving medieval find, taken from East Orchard Farm, St Athan. The Joneses acquired East Orchard Farm in 1756, but abandoned the building after stripping it of its dressed stonework. Further south is a battlemented watch tower of either 17th or 18th century design, thought to have been modeled on the one found at St Donat. It is believed the watch tower was constructed in two builds, and may have been founded on original ruined medieval stonework. Although it appears to be of 16th century design, it does not appear on the estate plan of 1622, but does appear on the plans of 1770 giving a wide window for its construction date. Due to its time scale and features, it is assumed to have been constructed during the medievalizing improvements carried out by Robert Jones III. It is constructed of local roughly coursed limestone rubble, which has been lime-plastered. It was given Grade II* listed building status in 1952, with the reason given a "an interesting example of a C16 and late C18 look-out tower and for its group value within the gardens of Fonmon Castle".
Interior
Notable features include the combined grand drawing room and library, designed by Thomas Stocking. Described by Newman as the "glory of Fonmon", the library, running east to west is lit by two Venetian windows, a stone one to the west wall and a sashed timber oriel window to the east. The room is divided into three sections, the largest central, with square end bays with segmental arches. There are trophies of the chase in the spandrels of the arches and arabesques and wreaths adorn the flat of the ceiling with an Apollo head in a sunburst at its centre.
