One such is Song Before the Sons of Llyr (poem 14 as translated by William F. Skene, 1868):
- Complete is my chair in Caer Sidi,
- No one will be afflicted with disease or old age that may be in it.
- It is known to Manawyd and Pryderi.
- Three utterances, around the fire, will he sing before it,
- And around its borders are the streams of the ocean.
- And the fruitful fountain is above it,
- Is sweeter than white wine the liquor therein.
The other is The Raid on the Otherworld (poem 30, ibid.):
- I will praise the sovereign, supreme king of the land,
- Who hath extended his dominion over the shore of the world.
- Complete was the prison of Gweir in Caer Sidi,
- Through the spite of Pwyll and Pryderi.
- No one before him went into it.
- The heavy blue chain held the faithful youth,
- And before the spoils of Annwvn woefully he sings,
- And till doom shall continue a bard of prayer.
- Thrice enough to fill Prydwen, we went into it;
- Except seven, none returned from Caer Sidi.
The poems are attributed to the 6th-century poet Taliesin but are of later date. Some attempts have been made to give the fortress a physical location, e.g. as the island of Grassholm off the coast of Pembrokeshire, but Caer Sidi is more likely to belong to the class of otherworldly forts and islands so prevalent in Celtic mythology.
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.
A castle with a spiral construction (as suggested by some in connexion with the spiral mazes found at Glastonbury Tor), or a 'revolving fortress' is unlikely.
- Alwyn and Brinley Rees, Celtic Heritage (London, 1961), p. 48