Sorry to be so slow in responding, but I've been offline for the past two days...

Glad you liked my “Artist at Wayland’s” image! It amused me, too, for a few seconds!

You say "I was very interested by your mention of a possible deliberate optical illusion along the sides. Do you have any more details?"

No, I'm sorry, I have no more details other than i saw it with my own eyes and haven't found anything else about it. It seems to me too perfect to be anything other than deliberate entasis.

Having studied classical (and indeed modern) architectural design this is a common phenomenon in the structures of the ancient civilisations of the mediterranean. Entasis lends more impact, drama, sense of bulk and deliberation to a structure and makes it easier to 'read' by anyone viewing/using it. Why should not entasis have been used in ancient western europe buildings and structures, too?

The Romans did it all over the friggin' shop and even adopted the practice in their style of letterforms: by adding a seriph to the end of a vertical stroke, the eye is given a 'full stop' and the letterform becomes easier to read from a distance. (Like on the the top of public buildings for example - all done for propagandist reasons!)

It is hard to know if it is definitively deliberate though because so many of these sites have been tampered with over the years.

It is something I frequently sit and consider at sites but I dont know of any other examples :-(