Some port calls don't involve leaving the ship: the ship itself becomes the lookout. Between Rüdesheim and Koblenz, the Rhine narrows for 65 kilometres into a gorge that locals simply call the Rhine Gorge. Hilltop castles, near-vertical vineyards and turreted villages follow one another almost without interruption, so much so that UNESCO inscribed this entire landscape as a World Heritage Site in 2002. Most river itineraries schedule this passage during the day, with commentary over the loudspeakers and passengers crowded onto the upper deck.

A river watched over by forty castles

From Bingen on, the scene takes shape: the Mouse Tower stands on its islet in the middle of the current, a former watchtower and toll tower with a legendary air. On the heights, Rheinstein, Reichenstein and Sooneck then file past, restored in the 19th century by Romantic-era Prussia, followed by Stahleck Castle above the rooftops of Bacharach. This corridor holds some forty castles and fortresses, an average of one every two kilometres: nowhere else in Europe is the density so high, a legacy of the days when every lord levied his due on river traffic.

The toll in the middle of the water

Off Kaub, a stone vessel seems anchored in the middle of the current: Pfalzgrafenstein, a fortified toll station set on an islet, once forced boatmen to pay up under the threat of the cannons of Gutenfels Castle, clinging to the hillside. It's one of the most photographed images of the route, with its white walls and slate roofs shaped like a prow, cutting through the current since the 14th century.

The Lorelei, at the river's narrowest point

Shortly after Oberwesel and its towered ramparts, the Lorelei cliff appears around a bend: more than 120 metres of slate above a passage where the riverbed narrows to less than 120 metres wide. Currents and reefs capsized so many boats here that legend invented a siren combing her golden hair to explain the shipwrecks; Heine's poem made her immortal. Ships slow down, the statue of the maiden appears on her jetty, and everyone searches for the top of the cliff, where a flag flies.

From Sankt Goar to Koblenz

Facing each other, Sankt Goar and Sankt Goarshausen keep an eye on one another beneath their fortresses: Rheinfels, the largest ruin in the valley, and the twin castles nicknamed Cat and Mouse. Then the valley widens toward Boppard and its great bend, before Marksburg, the only hilltop fortress in the area never captured or destroyed, whose bristling silhouette towers over Braubach. Vineyard terraces cling to the steepest slopes, worked by hand for centuries to produce renowned Rieslings. Finally, the neo-Gothic Stolzenfels Castle heralds Koblenz, where the valley ends at the Deutsches Eck, the point where the Rhine and the Moselle meet.

Making the most of the passage

• Allow two to three hours on deck: the scenery is continuous, with no lulls.
• Settle in early on the port or starboard side depending on the direction: the commentary points out each bank.
• Binoculars are useful for picking out battlements, coats of arms and winegrowers at work.
• The passage is done in both directions; downstream, the current noticeably speeds up the sailing.

When the valley opens up again

Past Koblenz, the river spreads out again and the upper deck gradually empties. What stays in your memory is a cliff sung by poets, a little fort planted in the middle of the water and dozens of towers watching for their ghostly boatmen. The Rhine Gorge can be crossed in half a day; it often takes years to stop thinking about it.