Ziegenberg
Found along Route 275 just west of Bad Nauheim, Ziegenberg looks unassuming.  In fact, I wasn't sure what I was looking at as I cycled past, but decided to snap the picture just the same. 

As it turned out, the castle was used as a headquarters for Heeresgruppe West under Generalfeldmarschall Karl Gerd von Rundstedt during the last months of World War II.  The castle was both headquarters and residence for many of the officers, and was the center of operations for Operation Wacht am Rhein, the famous Ardennes counteroffensive known as the Battle of the Bulge.  Hitler briefly commanded the forces from this location in December 1944.

Besides the castle, there was another barracks and headquarters building just to the north.  To see it, just take a right at the base of the castle on SchlossStrasse.
Allied intelligence picked up the location in early 1945, and the XIX Tactical Air Command decided to target the castle with an airstrike.  On 19 March, the XIX TAC hit Rundstedt's headquarters with several low level runs using 500 and 1,000 lb bombs with 8-11 second delayed fuzes.  Following the high explosive attack, another low-level run hit the castle with 150 gallon naplam bombs, at that time still a fairly new development to unnerve the most hardened combat veteran.  Two 1,000 lbers made direct hits on the castle, and the napalm burned out the remainder. 

Needless to say, Rundstedt was upset.  He stated later that "Allied bombers not only shattered our supply lines but they carried the war right home by hitting the headquarters at Bad Nauheim."  The castle was a complete wreck and its current state is hardly representative of its ancient construction.

Photos and data taken from "Tactical Air Operations in Europe" an after action report of the XIX TAC compiled in May 1945.  Originally "confidential," the report is now declassified.