The castle is believed to have remained under the ownership of just two families throughout its history; from Norman times, it was owned by the St Johns, and from 1656, by the descendants of Colonel Philip Jones.

[3] 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.

[1] 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.

[1] These early constructions are thought to have been undertaken by the St. John family (see Viscounts Bolingbroke), who were associated with the castle during the Middle Ages and continued as owners until 1656.

[1] It is believed that Jones then improved the rooms on the east range, and added a double-depth wing to the north side.

[1] In 2020, after the sale of the castle to a local businessman, new visitor facilities were added to the grounds turning it into a fantastic day out for all the family, including a history-themed attraction, Jurrasic Wales, Medieval Farm, Welsh Story Trail, Woodland Walks, Play Areas and more.

The stable has a fine polygonal stone chimney, which is a rare surviving medieval find, taken from East Orchard Farm, St Athan.

[10] Due to its time scale and features, it is assumed to have been constructed during the medievalizing improvements carried out by Robert Jones III.

[10] It was given Grade II* listed building status in 1952, with the reason given "an interesting example of a C16 and late C18 look-out tower and for its group value within the gardens of Fonmon Castle".

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.