Katz & Maus
Burg Katz ("Cat Castle") looks down on St Goarshausen from the river's right bank; Burg Maus ("Mouse Castle") sits above the neighbouring village of Wellmich on the same side, a couple of kilometres downstream. Their names aren't historical accident — they're an insult. When the Counts of Katzenelnbogen built their new fortress in the 1370s, they deliberately named it to overshadow the older castle nearby, which belonged to the Archbishop of Trier. Locals started calling the older one "the mouse" the newer cat was hunting, and the names stuck for over 600 years.
What you can actually see
Neither castle is open to the public in the way Marksburg or Rheinfels are. Burg Katz is privately owned and used for exclusive events; Burg Maus has been through several private-ownership eras and is not set up for regular tourist visits either. What the valley gives you instead is one of its best viewpoints: from the KD ferry or a riverside path between St Goar and St Goarshausen, both castles are visible in close succession, framed by the gorge's steepest section of the whole 65-kilometre stretch.
The feud behind the names
The rivalry wasn't just about naming rights. Both fortresses controlled river tolls on their respective stretches, and the Katzenelnbogen counts and the Trier archbishops spent much of the 14th and 15th centuries in intermittent conflict over trade routes, tolls, and territorial claims along this section of river — a smaller-scale version of the toll disputes that produced most of the valley's 40 castles in the first place.
Best place to see both
The clearest joint view is from the river itself, on the stretch between St Goar and the Loreley — a KD cruise or the passenger ferry between the two towns puts both castles in frame within a few minutes of each other. From land, a short climb above St Goarshausen toward Burg Katz gives a good look back at Burg Maus across the water, though neither building's interior is accessible.