Rheinfels
Started in 1245 by Count Diether V of Katzenelnbogen, Rheinfels grew over five centuries into the largest fortress on the entire Rhine — big enough to withstand a full siege by 4,000 troops in 1626 without falling. It took French revolutionary forces to finally beat it: in 1797 they blew up most of the complex rather than garrison it, leaving the sprawling ruin that covers the hillside above St Goar today.
Why it's the best ruin to actually walk through
Unlike most Rhine castle ruins, which are either roped off or reduced to a single wall, Rheinfels is large and stable enough to wander through freely. The grounds include a small museum with a scale model showing the fortress at its full extent, several courtyards, and battlements you can walk along with genuine river views. It's also the only castle in this guide with a hotel and restaurant built into part of the surviving structure, so you can eat lunch inside a 13th-century fortress wall.
The tunnels
Rheinfels' siege tunnels — dug to let defenders move troops and supplies without exposure to attackers above — are open to the public, and a section is left deliberately unlit. Visitors are told to bring a torch or phone light; it's a genuinely dark, low, narrow crawl in places, not a lit museum corridor, and claustrophobic visitors should skip it. It's one of the few places on the Rhine where you experience a castle's defences physically rather than just reading a plaque about them.
Getting there
Rheinfels sits directly above St Goar on the river's left (west) bank, a steep but short walk (about 10–15 minutes) up from the town centre — steeper than Marksburg's approach but shorter in distance. There's also a small car park closer to the castle entrance for those who'd rather not walk uphill from the river.
How it compares to Marksburg
Rheinfels rewards visitors who want to explore at their own pace with no guided-tour schedule to work around, and it's the better pick for anyone with kids old enough to enjoy tunnels and ramparts. Marksburg, covered in the Marksburg guide, is the better pick if you want to see furnished medieval rooms rather than open-air ruins.