Marksburg
Headquarters of the German Castle Association (Deutcheburgenvereinigung.)
Marksburg, if you have very little time to dedicate to castle hunting, should be your primary destination castle. This structure is in prime preserved condition, having been able to avoid destruction throughout its existence.
Positioned with a commanding view of its location along the Rhine River, it has been safe from assaulting forces. 
Marksburg is headquarters of the German Castle Association and houses a research library of many of Germany's 12,000+ structures that includes medieval castles, castle ruins, palaces, mansions and other historic locations. A tour of the castle will include rooms and museum pieces representing several eras from Germany's often explosive medieval history.
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Location
Rhineland Palatinate
50°16’18.97”N 7°38’56.83”E
Marksburg originated In the 12th century and remains as one of the best representative castles of the approximately 20,000 structures from the Middle Ages. Its location offers an additional bonus for castle hunters. It is in among the dozens of other castles that enhance both sides of the Rhine River enabling travelers to visit numeros sites in one day.
Please refer to "About" page for full map of Germany showing the location of the state.
The book Roads to Ruins
portrays many of the nearby castles.