The poem of Taliesin Preiddeu Annwfn contains the fullest description of the Briton “other world” that mythological literature can provide.
It has been collated by Charles Squire (1905) from four different translations of the text, those being of Mr. W. F. Skene, Mr. T. Stephens, Prof. John Rhys, and D. W. Nash.
[1] Squire also provides an interpretation of the poem:[1] Another few lines penned by Taliesin are sometimes connected to Caer Sidi, although they add little knowledge to what has already been stated in the verses above.
These lines are contained in a poem called "A Song Concerning the Sons of Llyr ab Brochwel Powys" (Book of Taliesin, poem XIV): This confirms that the "heavy blue chain [that] firmly held the youth" was the sea surrounding Annwn, the Welsh Otherworld, and it makes the point that its inhabitants were freed from age and death.
The Welsh translation of the "Seint Grael", an Anglo-Norman romance embodying much of the old British and Gaelic mythology, locates its "Turning Castle" (Caer Sidi), in the district around and comprising Puffin Island, off the coast of Anglesey.