A member of the OBOD message board has written this:
In France, Southern Germany, Austria and the Czech Republic we encounter a type of monument that was named Celtic Viereckschanzen.
This is a German word composed from "rectangle" (Viereck) and "entrenchment, redoubt" (Schanze).
A Viereckschanze is usually a rectangular area surrounded by a wall and a ditch, with one entrance which is never in the north. The entrances have been found to be in alignement with the ley line system.
There are approximately 150 Viereckschanzen known in Germany alone. They are dated to ca. 200 - 50 BCE.
To read more and see photos he has included see here.