Annwyn (or Annwfn)
The Celtic other-world. According to ancient belief, it might
be located either on or under the earth or the sea, or might be
a group of islands or a revolving castle surrounded by sea, and
Anima Mundi Encyclopedia of Occultism & Parapsychology 5th Ed.
58
was variously known as Land Over Sea, Land Under Wave,
or Caer Sidi (revolving castle). It was said to be a land of strange
beauty and delight, with a magic caldron having miraculous
powers. It is described in such works as the Book of Taliesin and
the Mabinogion. (See also Hell)
Sources
The Mabinogion. London J. M. Dent, 1949.