As I progress through ‘The Spoils of Annwn’ it is my growing intuition that imagistic links* suggest Taliesin is not referring to a series of different forts raided by Arthur and his men but to one otherworldly fortress by different names.
Under this tentative interpretation, verse four takes us from the theft of the cauldron and escape from Annwn back to the beginning of the action. Taliesin says:
‘I don’t rate the pathetic men involved with religious writings,
those who hadn’t seen Arthur’s feat beyond the Glass Fort:
six thousand men were standing on its wall;
it was hard to communicate with their watchman.’
Taliesin is mocking ‘pathetic men’ (monks) for being unable to see beyond Caer Wydyr: the Glass Fort. This is the fourth fortress named in the poem but, I intuit, the first to be approached.
In modern Welsh, gwydyr means glass. The image of the fort as glass: clear, see-through, near-invisible, is deeply evocative of its otherworld nature. Its walls, with room for six thousand men, are extensive.
It is possible to think of these glass walls as representative of the boundary between thisworld and Annwn. To penetrate beyond requires an invitation from Annwn’s deities, cunning, or a good deal of brute force.
Much speculation surrounds the incommunicative nature of the watchman. In other poems and stories such as ‘Arthur and the Porter’ and Culhwch and Olwen, the watchmen/gatekeepers are communicative. Questions must be answered and conditions met to enter the fortresses of otherworldly persons.
The watchman’s incommunicability has led some scholars to suggest he and perhaps the six thousand men on the walls are risen dead. In The Second Branch of The Mabinogion, dead Irishmen thrown into the Cauldron of Rebirth rise able to fight but unable to speak.
Another possibility is the guards are the spirits of Annwn who are ruled by Gwyn ap Nudd. They are perfectly able to speak: in The Life of St Collen, Gwyn’s watchman courteously invites St Collen into ‘the fairest castle he had ever beheld’.
The ominous silence of the watchman and six thousand men could stem from the fact three loads of warriors from Arthur’s warship have just landed outside their home, fully-armed, with their eyes glinting with lust for Annwn’s treasures.
***
Glass fortresses are a recurrent feature in Celtic literature and are often the abode of otherworldly rulers. This leads me to suspect there was once a deep, underlying mythos surrounding the Head of Annwn and his otherworldly fort which has gradually been lost.
A very close parallel with Caer Wydyr can be found in Nennius’ History of the Britons. Thirty ships of Spaniards sailing to Ireland find in the midst of the sea ‘a tower of glass, the summit of which was covered with men, to whom they often spoke, but received no answer.’ When they attack the tower, all but one of their ships are swallowed by the waves.
In The Life of St Gildas, Glastonbury is described as the Island of Glass. Caradog of Llancarfan says ‘Glastonia was of old called Ynisgutrin, and is still called so by the British inhabitants. Ynis in the British language is insula in Latin, and gutrin (made of glass).’
This is echoed by Gerald of Wales in Speculum Ecclesiae. Glastonbury ‘used to be called “Ynys Gutrin” in the Welsh language, that is the Island of Glass, no doubt from the glassy colour of the river which flows around it in the marshland.’
Glastonbury is the abode of Gwyn ap Nudd and Melwas who are both abductors of important female figures and riders of the famous water-horse ‘The Black of the Seas’**. Melwas keeps Gwenhwyfar imprisoned on the Island of Glass.
In Sir Orfeo, the Fairy King abducts Orfeo’s wife, Heurodis, and takes her to his crystal castle. Its vivid description provides clues to the appearance of Caer Wydyr:
‘Amid the land a castle tall
And rich and proud and wondrous high
Uprose, and all the outmost wall
Shone as crystal to the eye.
A hundred towers lit up the sky,
Of diamond all battled stout;
And buttresses rose up near by
Arched with red gold and broad about.
All the bonsour was carved in stone
With every beast and every wight,
And all within the castle shone
And sparkled with unearthly light.’
***
In The Four Branches of the Mabinogi, Will Parker suggests the imagery of ‘the Indigenous Underworld of Annwfn’ stems from the ‘memory of the riches of the Romano-British civilian zone’.
The Romans brought glass-making to Britain and it seems likely memories of their cosmopolitan ways of life, fine clothes, and wine fed into conceptions of Annwn, which later became known as Faery.
A fascinating blend of Brythonic and Roman influence can be found in the list of ‘The Thirteen Treasures of the Island of Britain’ ‘that were in the North’:
‘Drynwyn… the Sword of Rhydderch the Generous’, ‘The Hamper of Gwyddno Long-Shank’, ‘The Horn of Brân the Niggard’, ‘The Chariot of Morgan the Wealthy’, ‘The Halter of Clydno Eiddyn’, ‘The Knife of Llawfroedd the Horseman’, ‘The Cauldron of Dyrnwch the Giant’, ‘The Whetstone of Tudwal Tudglyd’, ‘The Coat of Padarn Red-Coat’, ‘The Vat and Dish of Rhygenydd the Cleric’, ‘The Chessboard of Gwenddolau son of Ceidio’ and ‘The Mantle of Arthur’.
All these treasures possess magical qualities which are suggestive of their otherworld origin. They are also of interest because in the marginalia of the list in Pen. 147 (1566) we find the story of how Myrddin managed to acquire them and retreat ‘to the Glass House’ (ty gwydyr).
The traditional location of Myrddin’s glass house is Bardsey Island. Patrick Ford says ‘In a version of… “Treasures of the Kings of Britain”… Mad Merlin took all these from the city called Caerlleon-on-Usk to the House of Glass in Bardsey Island’.
In Celtic Remains, Lewis Morris says ‘In Caerlleon on Usk there was a museum of rarities in King Arthur’s time, which Myrddin ap Morfran, the Caledonian, upon the destruction of that place, carried with him to a house of glass on the Isle of Enlli or Bardsey…
This house of glass, it seems, was the museum where they kept their curiosities to be seen by everybody, but not handled; and it is possible Myrddin, who is said to live in it, was the keeper of their museum at the time’.
The shift from treasures used by their owners (some stolen by Arthur: Gwyddno’s Hamper and Dyrnwch’s cauldron) to their placing in a glass museum as relics is an interesting one, which reflects that they have fallen out of use and become part of our cultural heritage.
It also shows we have become cut off from their magic. As Christianity replaced Romano-British paganism, the doors to Annwn were sealed. Mad Myrddin became the uncommunicative watchman of the glass walls.
Caer Wydyr and its people have been forgotten and we have since then built our own Glass Fortresses: Crystal Palaces, Arcades, department stores, shopping malls, and stocked them full of the treasures of thisworld.

The Crystal Palace, London, Great Exhibition 1851, Wikipedia Commons
Yet the call returns to journey to Annwn, to see beyond the Glass Fort to the feats of Arthur, to their consequences reflected like mirror images in the years between then and now.
*Caer Vedwit and Caer Rigor are described as having four quarters/corners/turrets/pinnacles/ peaks and Caer Vandwy is referred to as the Fortress of God’s Peak. The sea beats around the turrets/pinnacles of Caer Siddi.
**In Culhwch and Olwen the only horse Gwyn can hunt Twrch Trwyth ‘King of Boars’ with is Du y Moroedd ‘The Black of the Seas’. In the opening lines of The Dialogue of Melwas and Gwenhyfer, Melwas introduces his horse before himself
‘Black is my steed and brave beneath me
No water will make him fear
And no man will make him swerve.’
It is notable that in The Conversation of Gwyn ap Nudd and Gwyddno Garanhir, Gwyn also introduces his horse (although here it is Carngrwn) first.