The Tale of a Cloth: Fortresses Facing Germany during World War I

Today's (this week's? turned out to be this year's!) wondering stems from trying to figure out what the map to the right is. Unlike what one may expect on a quick glance, it is not a map of Europe, but mostly north central Europe: eastern Germany, western Russia, and northern Austria-Hungary. In addition, the big bold-faced names are not those (with some exceptions) of major European cities. So what is it?

It is part of of what had been a fairly sizable collection of World War I materials saved by my great aunt. So, although undated, I have always known that it as some World War I German thing. In deciding to unburden ourselves of more stuff, I needed to figure out finally what the map is all about, so this is where I wandered.

The facts: the map is made of some sort of line, and it measures 16 1/2" x 16 1/2" (42 cm square). In looking at closely, I see that there is both a key (for distance) at the top and (perhaps) a title: the word "Festungen". So first things first, is the Google translate which gives me "Forts"; in parallel, Wikipedia takes me to Fortification. 

This lists fortresses (Festungen) in existence in eastern Germany, western Russia, and northern Austria-Hungary, presumably at the beginning of the war. Undated, published by Verlag Sander & Gronau, Hamburg.

So now that I can see that each of the symbols denotes a fortress, my wandering takes me to each of them (roughly going from North to South) on the map. The big question, however, will still be why this map and who was it for!