[LadyDoc]

there is a BBC program on this linkand another one on Offa's dyke in the series "search for the Dark Ages".

Alas, Glastonbury is not manmade, although there are huge hills there.

ALSO google to find the program "Standing with stone".

[gleeaikin]

The jungles of Yucatan are also known for long elevated causeways. Then there is the Great Wall of China. These all provide similar advantages.
1) Impressive to the common man and invaders.
2) Good to move fast along the top to reinforce areas with problems or attacks.
3) Good view over hills and jungles, while traveling and from spaced towers and barracks.
4) Above flooding and snow drifts.
5) Unambiguous boundry lines.