Bala Lake
Long, long ago, there was a fertile valley where now roll the waters of Bala Lake.
“At last he reached the top of a hill, some considerable distance ‘from the palace”.... Although the story isn’t specific – mythical legends, eh? – I guess it’s not utterly unreasonable to suppose Moel y Garnedd, overlooking Llyn Tegid (Bala Lake), may have been the inferred destination of the harper... being an old man and presumably not up to a trek up any mountain proper:
Played on nicely in Susan Cooper's "The Grey King", where the playing of a harp (itself taken from a secret chamber accessed from Craig yr Aderyn) awakes the twelve Sleepers to come and fight for the Light.
Went paddling in Bala lake last year - it was cold! :)