Russian troops appear to be making tactical advances just northwest of Avdiivka toward a so-called "fortress belt" of Ukrainian ... to stabilize sections of the front and support penetration ...

and "The Fortress," about the siege of Przemyśl on the Eastern Front. You can find Watson's books here: "Ring of Steel: Germany and Austria-Hungary at War, 1914-1918": www.hachettebookgroup.com ...