Do you know why it is called "Castell Nedd" in Cymraeg? Doesn't "Castell" mean "castle"?
Because there is a castle there (so you're quite right, Robert!), and it's on the River Neath (Afon Nedd).
Why does the translation of 'Castell Nedd' not accept Castle Neath, only, Neath?
[Why include the word castle?]
The legal Welsh name for 'Neath' is 'Castell-nedd', so we are slightly wrong here, we need to add the hyphen.
If someone wanted to refer to the castle there they would write 'castell Castell-nedd' (Neath castle)
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).