Here is a photo of the interior of a German artillery shelter near Andignicourt (Aisne). It is from a US AEF report on dugouts and concrete shelters. As you may know, shelters such as these were built in 1915 or 1916. With the construction of the Hindenburg Line (Siegfried-Stellung), the German army moved away from shelters for artillery, instead preferring to build many prepared firing sites and then moving the artillery batteries around to avoid counter-battery fire.