Penrhyn Rhionydd
Arthur’s northern capital, according to the Triads.
It may have been in Scotland. Its name suggests that it is a cape, or the tip of a peninsula.
Notes
The Welsh pen, in geographical names, means the highest part or the extreme end, as of a mountain, a field, or a meadow. In Scotland there is ben, “a mountain;” Gaelic cen or cenn with the same meaning as pen and ben. Some Scottish places are Benmore (Penmawr), “great mountain;” Pencraig, “top of the rock;” Penpont, “end of the bridge,” and so on. European names gives us information about the earlier Celtic settlements, such as Pennine, Penne, Penmark, and Apennines.
Source
Triads of the Island of Britain (Welsh ”Triads”) | 11th century to 14th century