I first read about shaft burials in T D Kendrick's book on Druids, I think. It was more than 20 years ago when most druid books were lightly sidestepping around the issue of the celts making sacrifices, especially human ones. (If they weren't just outright claiming it was all prpaganda and not true).
Perhaps he's been proven to be wrong, in the meantime. Or people think the purpose was simply to make a well shaft, perhaps one which emulated the watery grave you get in bogs. Perhaps there's new evidence connected to these, and I need to update my source material. I ind myself doing that a lot here, LOL. A quick google still shows lots of pages talking about shaft burials, though not with the tree in them, so far. MAybe that was very rare.
Anyway, I seem to rememeber being astounded that some shafts were up to 120ft deep, and occassionally had been found to have had an upside down tree placed in them. At the bottom, animal, votive, and occassionally human remains were found. The book stated that you have to consider that your basically humanitarion, non-sacrificing druid did not exist, and shafts with trees in them might be part of such rituals.
If I placed myself inside that story, I'd imagine the upside down tree as symbolic of climbing the tree of life into afterlife. Perhaps it was a path guaranteed to place you in the cauldron of rebirth. The spiral wooden fence being built last, so you could visit the ancestors, human and otherwise, buried there, and yet keep them contained away from the living world, where their spirits might make mischief.
I probably only remember it cos it gave me nightmares of being drugged, helped to climb down a tree, and being buried alive.
http://www.sheshen-eceni.co.uk/images/tgc_001.JPG
