The tower on top of Glastonbury Tor is a ruin standing because an earthquake took down what came before it, one of very few English earthquakes recorded in detail.
The building history
A church dedicated to Saint Michael stood on the Tor by the twelfth century. In 1275 an earthquake, an unusual event for England and recorded in near contemporary chronicles, brought the tower down. The replacement tower visible today dates from the fourteenth century; the nave beside it was demolished after the suppression of Glastonbury Abbey in the sixteenth century, leaving only the tower standing on the summit.
The terraces, argued over
Concentric terraces cut into the Tor's slopes have been read two ways. Geoffrey Russell proposed in 1968 that they form a Neolithic ritual labyrinth, a walkable maze of considerable antiquity. Most archaeologists instead treat the terracing as strip lynchets, medieval or later agricultural ploughing marks adapted to a steep hillside, a mundane explanation that the labyrinth reading has not displaced in the archaeological literature even as it thrives outside it.
Avalon comes later than it looks
The Tor's identification with Avalon is a medieval addition. Monks at Glastonbury Abbey announced in 1191 that they had found the graves of Arthur and Guinevere on the site, a discovery that arrived conveniently soon after a fire had damaged the abbey and its finances needed rebuilding.