Fonmon Caste is a mansion that incorporates substantial elements of a medieval masonry castle, including a late twelfth century keep and work of the thirteenth to fourteenth centuries. The castle was much added to in the sixteenth century, with a major refurbishment around 1760, and minor modern additions and alterations.

It is speculated that a timber castle was built on this site soon after the Norman Conquest of South Wales, the first stone building added in about 1200. This would have adjoined or been within the timber enclosure, but there is little physical evidence, apart from the changes in level in the gardens and outworks and the 'Watchtower'. The first surviving stone build is well away from the edge of the ravine, and is the section on the left of the entrance court, running east-west and containing the Drawing Room in what was the first floor hall of the castle. This section has the thickest walls of the present building, but since it is small, approx. 8m x 13m, it is possibly only a part of what was in use until more of the stone castle was built.

This was the east curtain which extended the existing range to the edge of the precipice and then turned the south-east corner to include the south range. The rooms within the east curtain, now the Stairhall, appear to have been created by Colonel Jones in the 1660s, with both the west wall and the roof dating from then, but it would probably have had a building against it previously. The thickest section of these walls are along the ravine edge where attack would be least likely, so the more vulnerable south and west approaches to the castle must have been defended in some further way not now apparent. This second phase of stonework, with both square and rounded towers, must be from fairly early in the C13, beforr the more vulnerable square corner tower fell from favour. It was probably built by the St. John family, who were the owners later in the Middle Ages and until 1656.

The open quadrangular form of the C13 castle with apartments set against a curtain wall would strongly suggest that it was intended to be a complete quadrangle, but this appears never to have happened. The next major addition seems to be a short, north wing built in the C16 over a characteristic barrel-vaulted semi-basement. The castle was undamaged in the Civil War, the St. John family being Parliamentarians, but they later fell on bad times financially and in 1656 sold their Glamorgan estate to Colonel Philip Jones whose successors still own it today. The Colonel appears to have been responsible for the addition of the double depth wing on the north side of the castle and the rooms of the east range, but some of the internal decoration may date from after his death in 1674 and be attributable to Oliver Jones 1678-85.

The next major addition, representing its present internal character, was the work of Robert Jones III. He married Jane Seys of Boverton in 1762 and began the castle improvements by employing Thomas Paty of Bristol. This firm had done similar makeovers to houses such as the C16 Stoke Park north of Bristol in 1760-4, and they supplied the overall battlements and render to enhance the castle look while at the same time putting in a regular display of sash windows to improve the light, and remodelled the interior for comfort and elegance. This involved breaking through walls to increase the size of rooms, thus forming the Stairhall and the Drawing Room, and making further improvements to the service end following what had already been achieved in the late C17.
The C19 saw the estate in decline and little was done to the castle except for the addition of the entrance porch and the extension to the south wing between 1840 and 1878; the extension appears to be shown on the Tithe Map of 1841. There is a possibility that the C19 alterations may have been more considerable, with the mid C18 pastiche on the staircase and in the Dining Room dating to this time for example, but this is uncertain. The castle passed by marriage to Sir Seymour Boothby in 1917 and his grandson still lives there.

(Source; Cadw listing database)

S Fielding, RCAHMW, 4 December 2006.