I have been taking the Welsh course for one and a half years now, and the spelling of Castell Nedd never had a hyphen. But now it suddenly has!
Beth yw'r sillafiad cywir?
We corrected it in this new version of the course - Castell-nedd. I think the original course had it both with and without the hyphen as both are seen, but the version with the hyphen is the correct one.
See the course notes. The basic form of the name is Castell-nedd. Welsh has a system of initial consonant mutations. One if those is a nasal mutation caused by yn when it is used as a preposition meaning 'in' or 'at' - an initial letter c- changes to the two letters ngh-. Also, in front of a word starting the letter ng-, yn becomes yng to match the sound:
- yn + Castell-nedd => yng Nghastell-nedd
A mutation is not usually applied to a word unless the reason for the mutation is present, though. In this case, Nghastell-nedd would not be used on its own without the yng.
The various mutations arr explained in the course notes for the sections of the course where they are first introduced.
Except -- just to be extra picky -- it's Castell-nedd, with a lower-case n. :)
That rule applies to all place names where the hyphen is there to indicate stress. (See the Welsh Language Commissioner's Guidelines for Standardising Place-names in Wales).