As rmcode explains (above), the anglicized version of Castell-nedd is just Neath. See the road sign in the picture at https://ichef.bbci.co.uk/news/720/cpsprodpb/13ED1/production/_96071618_capture1.jpg
In Welsh, Nedd (on its own) is the name of the river which flows past the town. In the 8th century the Romans built a fort on the opposite bank of the same river, and called it Nidum. You can see a picture and a short account in Welsh of the "more recent" (13th century!) castell Castell-nedd at http://www.walesonline.co.uk/special-features/24-llun-syn-profi-mai-13117151 (scroll down to castle no 18).