The Kingdom of Tylwyth Teg was the kingdom of the Fair Folk. It was located under The Black Lake in the Eagle Mountains of northern Prydain, north of Medwyn's Valley. It was home to Doli and many other dwarfs and different varieties of Fair Folk, including the Children of the Evening and the Lake Sprites, and ruled over by the gruff but benevolent King Eiddileg.
The kingdom appears only in The Book of Three, where it is described as a vast and beautiful subterranean land.