I am building a diorama of a German artillery position in the french countryside. There is going to be a dirt road running through it that cuts through a hilly area. It needs retaining walls along it but I am not sure what to use. Any idea's ?

Retaining walls on the sides of the road?
Plasterboard where you scrib in some nice stonework.
One by one real big stones (pieces of plaster).

Depends on height, slope, and so on

Claude

Cork is another good method as I would recommend adding pillars to the wall if its long and/or high. Scribing can work if you have the shape. See here for some ideas on casting/scribing and also for walls from cork.