Glastonbury is the archetypal glass fortress. The name derives from Glaestingbyrig - town of the Glaestings, but it has a 'sounds like' assocation with glass, due to its old form of 'Glassenbury', the '-en' suffix now being little used in English. (Think of woolen, golden... these are about the only surviving examples).
Ynys Witrin (or sometimes Gutrin or Wydrin) means 'Isle of Glass'. In Medieval romance, this was the Otherworld, where lived nine maidens. The link is made explicit in the tale of Melwas and Guinevere, which first appears (I think!) in the c.1130 Life of Gildas. (I'd check that before using if I were you!)
There is also a glass fort in Preiddeu Annwn:
Ny obrynafi lawyr llen llywyadur
tra chaer wydyr ny welsynt wrhyt arthur.
Tri vgeint canhwr aseui ar y mur.
oed anhawd ymadrawd aegwylyadur
tri lloneit prytwen yd aeth gan arthur.
namyn seith ny dyrreith ogaer golud.
(I merit not the Lord's little men of letters.
Beyond the Glass Fortress they did not see the valor of Arthur.
Six thousand men stood upon the wall.
It was difficult to speak with their sentinel
Three fullnesses of Prydwen went with Arthur
Except seven none rose up from the Fortress of Guts (Hindrance?).
This verse is surprisingly similar to a stanza in the Historia Brittonum:
"Long after this, the Scots arrived in Ireland from Spain. The first that came was Partholomus, with a thousand men and women, these increased to four thousand; but a mortality coming suddenly upon them, they all perished in one week. The second was Nimech, the son of …..who, according to report, after having his ships shattered, arrived at a port in Ireland, and continuing there several years, returned at length with his followers to Spain. After these came three sons of a Spanish soldier with thirty ships, each of which contained thirty wives; and having remained there during the space of a year, there appeared to them, in the middle of the sea, a tower of glass, the summit of which seemed covered with men, to whom they often spoke, but received no answer. At length they determined to besiege the tower; and after a year's preparation, advanced towards it, with the whole number of their ships, and all the women, one ship only excepted, which had been wrecked, and in which were thirty men, and as many women; but when all had disembarked on the shore which surrounded the tower, the sea opened and swallowed them up. Ireland, however, was peopled, to the present period, from the family remaining in the vessel which was wrecked. Afterwards, others came from Spain, and possessed themselves of various parts of Britain."
This of course, is known to be 8th century; I'm not sure on the dating for Preiddeu Annwn.
Hope some of that stuff is useful
