Behind the fences which you are not able to access are the origins to World War Two Royal Ordnance Factory, most of which is now an industrial estate. The factory employed 40,000 people at its peak and supplied and filled naval shells. It connected to the Barry Railway via a siding and served underground magazines built into Brackla Ridge, for secure storage.
Far more information can be found online by searching for : Brackla Ridge to hear about the areas fasinating history
Well done to sniffadogz for being first to find