Janne,
The wall is supposed to represent the wall built by the Cherusci to hide behind and surprise the Romans. It is described in any decent book about this event, and there actually is archeological evidence for it. If you like, I can give you some references, let me know.
I must admit I am not particularly fond of this set. In the references, the wall was an elevated earth wall, with wooden top and/or cover with branches. The Cherusci stormed down on the Romans. Here, it is depicted as a log wall with mud cover. Besides, the depicted Cherusci hardly seem to hide behind it. I find the figures quite nice, but I would redo the diorama.
IMHO the set "Surrounded" is the best of the series. More suspense per square cm.
Nice painting by Doug Cohen by the way.
Cheers,
Adrian