Mynydd-y-Gaer is a hill that sits on the boundaries between the South Wales communities of Baglan (south-western quarter), Cwmavon (south-eastern quarter) and Briton Ferry, (northern half), all within Neath Port Talbot county borough. The summit, at , has grassland fields subdivided by dry stone walls. Foel Fynyddau lies 2 km to east. To the south is the coastal plain of the Bristol Channel. To the West is the Vale of Neath. To the north is the Crythan Brook and the town of Neath. It has numerous prehistoric monuments, and evidence of occupation in medieval times, as well as 19th and 20th century coalmining.