Straddling the lines that connect our world with Annwn, Caer Sidi is prison to countless restless souls, souls trapped there until the day when Apocalypse brings his wrath down upon the land of the living.
Caer Sidi (or Caer Siddi or Caer Sidydd) is the name of a legendary otherworld fortress mentioned in two Middle Welsh mythological poems in the Book of Taliesin.
The precise meaning of the name 'Sidi' in Caer Sidi is problematic (caer means "fort", "fortress", "stronghold"). One possible meaning of Caer Sidi is the 'turning fortress', but it is more likely to mean the 'Fortress of the Zodiac', as sidydd means "Zodiac" in modern Welsh.
Explanation: Caer Sidi is Irish Cathair Sídhe written phonetically in Middle Welsh. The name means 'Fortress of the Fairies' and is a reference to the Tuatha De Danann. It was also known as Caer Ochren (alias Achren, Aghran), a name nowadays spelt ‘Aghrane’ or ‘Aughrane’. This is the Achren of 'The Battle of the Trees', a fictitious battle whose site is known.
Caer Sidi is the Regia Altera shown on Ptolemy’s map of Hibernia, and was the royal abode of Mallolwch, prehistoric king of Ireland who wedded Branwen, sister of Bran in the Mabinogi legend Branwen ferch Llyr. It is now known as Abbeygrey, and its remnants can be seen on google maps by googling: Monasternalea.
Caer Sidi is a wheel-like (i.e. circular) site in which at least two medieval monasteries were built. One of these was raided by Welsh Normans in 1177 and a bundle of manuscripts of poetry and legends was seized. These legends have become known as the Mabinogi, and the seized poetry is promulgated as that of a fictitious Taliesin. The account of the raid is described in the Middle Welsh poem ‘Preiddeu Annwn’, the author of same being a gaelic poet who followed Milo de Cogan (called ‘Arthur’ in the poem), leader of the raiders, into the abbey, and evidently then emigrated to Wales.