Not to be confused with Pen Twyn Glas in the Black Mountains.Mynydd Twyn-glas is a sprawling 472-metre-high hill extending into Caerphilly and Torfaen county boroughs in South Wales. It lies southwest of Pontypool, west of Cwmbran and east of Newbridge and taking the form of a rolling plateau the margins of which are defined by steep slopes, many of which are cloaked with conifer plantations.