Seven Holy Islands
The “Isle” of Glastonbury and the hills (or rises) Bride’s Mound at Beckery, Meare (also called Ferramere), Godney, Barrow Hill (between Panborough and Bleadney), Marchey (also called Martinsey), Nyland (also called Andersey or Andrewsey) since Saxon times belonged to Glastonbury Abbey and, along with the “Glastonbury 12 hides” were named as exempt from taxation in Magna Carta. These are the “seven holy islands”. From the fifth to the tenth century, the five other than Glastonbury and Beckery probably were true islands wholly surrounded by the Brue and Axe rivers and their channels, tributaries and distributaries in a marshy landscape. All five are surrounded by land below today’s 10 metre contour above mean sea level, “which indicates the approximate extent of former periodic flooding”; much of it below the 5 metre contour. The Beckery-Glastonbury peninsula is similarly surrounded except along the Edgarley Road ridge to its east which at its lowest is 13 metres above sea level. According to Professor Stephen Rippon (http://humanities.exeter.ac.uk/archaeology/research/projects/title_90439_en.html ), works to control water flow into the Axe and Brue valleys from the east (thus beginning their journey to their present state of dry land) were undertaken from the tenth century onwards.
Spiritual Progress Through the Islands of Somerset Summerland
A straight (ley?) line runs from Nyland through Barrow and Godney to the Old Church of Glastonbury Abbey. According to the esotericist author John Michell, the pattern of the seven holy islands mirrors that of the stars of the Great Bear. It can be speculated that the Abbey valued the seven islands in sacred trust because they were previously held sacred by the Druids; and indeed that they, the holy rivers Brue and Axe which surround them (nominally equivalent to, and perhaps derived from, the holy rivers Varuna and Asi at India’s Varanasi), and the holy City of Light (glas), were the original Summerland, the earthly symbolic representation of the heavenly Otherworld, from which Somerset got its name. 
It is possible that each of the islands symbolises an aspect of the initiate’s spiritual journey to enlightenment. Some islands offer a clear suggestion of such symbolic meaning: Meare, once famous for its now-drained Meare Pool, for stilling the mind as in meditation; Andrewsey, for the “third eye” which yogis say is the door to an inner world where you experience transcendent love, light, and joy; Martinsey, named for St. Martin the West’s pioneer of withdrawal from the world like the Desert Fathers of Egypt, for renunciation; Beckery, which means “beekeepers’ island”, for the sweet honeyed “taste” of the nectar of divine bliss as the devotee crosses Pomparles Bridge, the “perilous” bridge (also known to all fairytale heroines) of letting go of all vestiges of ego, entering oneness with Spirit – and advances to full enlightenment, symbolically the city of light, Glas[ton]bury. Possibly Godney, God’s isle, was the symbolic island for devotion and prayer; and Barrow, alone of the seven set in a narrow water- channel between two proper hills (Wedmore and Wookey), the symbol for the self-discipline needed to overcome all barriers to spiritual attainment.
It may be no coincidence on this basis that if the Seven Holy Islands mirror the Great Bear constellation, Andrewey corresponds to the only star that can fairly be called the beginning of the constellation, and the line from Beckery to Glastonbury corresponds to the line that points to the Polestar.
Glastonbury’s Seven Holy Islands and the Seven Kaers of Preiddeu Annwn
It is possible to find a correspondence between these putative identifications of the symbols of the Seven Holy Islands and the seven “kaers” (royal citadels) in the poem attributed to Taliesin, Preiddeu Annwn. The poem in masked language expresses the initiate’s spiritual journey to find “the treasures of heaven, hidden within” (which is what I see as the true meaning of the title).
Kaer Sidi (line 10 of Preiddeu Annwn) is the ultimate spiritual destination, the state of enlightenment, corresponding to the Isle of Glastonbury. Nyland, considering its shape, could be Kaer Pedryuan, the “four-cornered” kaer (line 12). Kaer Vedwit (line 22), the citadel of wisdom, is on this schema Meare, the pool of stillness centred in meditation which enables the devotee to contact wisdom. Kaer Rigor (line 28) is the very highly spiritually advanced state of stillness in which the body is literally absolutely immovable and as if non-functioning (c.f. rigor mortis), known to yogis as sabikalpa samadhi, in which yogis say the pure bliss of God is felt, the last step on the initiate’s spiritual ascent before achieving enlightenment, so by both location and symbolic meaning this has to be Beckery. Barrow might symbolically be Kaer Golud (line 34), one translation of which is the citadel of “hindrance or impediment”, with Godney being Kaer Vandwy (line 42), the citadel of prayer. Finally, Kaer ochren (line 48), the “ochre-coloured” citadel has to be Martinsey, the isle of renunciation, for which the colour ochre, in which swamis are robed, is anciently the symbol.
 Rahtz and Watts, Glastonbury: 2009, p29.
 I have seen no evidence that the modern Welsh name for Somerset, gwlad yr haf, meaning “the summer land”, has any deep antiquity. But to the east of Glastonbury, cut by Edgarley Road less than half a mile from its lowest point, is Ponter’s Ball, a human-made earthwork obstacle that could have been used as a checkpoint to island Glastonbury from outsiders. Rahtz and Watts (ibid, p30) mention the possibility “that it was the eastern boundary of a great Celtic sanctuary around the Tor”. The road cuts Ponter’s Ball at a hamlet called Havyatt, which means “summer gate”.