One of the oldest settlements in Porthkerry is the Iron Age promontory fort known as The Bulwarks, a 4.1 hectares (10 acres) site much of which is now wooded.

[1] The Bulwarks, which consisted of three closely spaced overgrown banks fronted by ditches with the final side facing the cliffs to the south, were occupied well into the period of Roman occupation.

This would suggest that the cult was rife in Porthkerry and other parts of the Vale of Glamorgan, including near by Penmark[3][self-published source] which has similar engravings on pews and benches.

On 28 November 1831 the vessel The Nepture, sailing from Newport to Wexford in Ireland, struck a rock and sank off the coast of Porthkerry.

The western end used to be accessible to light vehicles from Rectory Hill, north-west of Porthkerry viaduct, via an unmetalled lane from the Rectory House at valley bottom (later Egerton Grey Country Guest house, opened in 1988 but closed in 2010) but later metalled as a cycle/pedestrian way to join the metalled road near Porthkerry railway viaduct and public car park.

[8] The land was acquired by the Romilly family in 1812 to build a country house, and cottages, stables and a sawmill for local workers.

The park was occupied by British and American forces during World War II in the approach to D-Day, and earthworks and defences were built along the coast.

[8] The park is particularly noted for the Porthkerry Viaduct, crossed by a railway originally used to transport coal from some of the South Wales Valleys north of Bridgend to Barry Docks port in the late 19th and early 20th centuries.

[12] On the northern side of Porthkerry Park there was a small hamlet named Cwmciddy (meaning Valley of the Black Dog in Welsh).