I did a bit of research on wetlands for my suggestion thread and thought that you might be interested in some reference pictures for different kinds of wetland. You could use them for the world and battle map, if you intend to tweak them or expand on the art. Europe basically used to be covered in forest and much of its land consisted of wetlands, making them great for evoking an older Germanic flavour. The writings by Tacitus are also rather interesting for this. Though clearly containing much propaganda, they give some nice descriptions of the land back then.
Most of the pictures are from Eastern Europe, since the land there tends to be less cultivated and relatively closer to what medieval German lands would have been like. I kept them as links, otherwise this post would get waaayyy too big. ;)
This is the Nienwholder Moor, Germany.
Another Polish swamp, this time near Sosnowica.
The Wadden Sea, Netherlands.
An overview of Saeftinghe, Netherlands.