Another site added that gives me an excuse to extol the virtues of Susan Cooper’s “Dark Is Rising” sequence. The fourth part, The Grey King, features Llyn Mwyngil at its climax, where six knights (the “sleepers”) rise from the waters of the lake to overcome the power of the Brenin Llwyd, the Grey King of Cadair Idris.
The lake is also referred to in one of the poems that feature through the sequence: “By the pleasant lake the sleepers lie”.
Nearby hillforted Craig yr Aderyn features in the book as well.