St. Goarshausen lies at the foot of two 14th-century castles whose names, Katz (Cat) and Maus (Mouse), reflect but one of the many power plays on the Rhine in the Middle Ages. Territorial supremacy and the privilege of collecting tolls fueled the fires of rivalry. In response to the construction of Burg Rheinfels, the archbishop of Trier erected a small castle north of St. Goarshausen to protect his interests. In turn, the masters of Rheinfels, the counts of Katzenelnbogen,
built a bigger castle directly above the town. Its name was shortened to Katz, and its smaller neighbor was scornfully referred to as Maus. Both castles are closed to the public.