But it's also because its glorious weatherbeaten stone walls topped with jagged battlements, and graceful towers with colourful pennants flying boldly from the top, perched on a rocky outcrop soaring 80m above the river, is just the way I'd always imagined a castle would look.
The guidebook I had bought from a riverside stall in the city of Mainz - The Long Rhine Tour - agreed that the Rheinstein was "one of the most beautiful castles along the Rhine" and also revealed that it has some great stories to go with its good looks.
Like many of the castles on the river it was built about a thousand years ago as a customs fort to ensure passing cargo boats paid their tolls. For a while in the late 13th century it was a base for the Holy Roman Emperor, Rudolph von Habsburg, as he battled to establish himself against rival claimants.
In 1823 it was acquired by Prince Friedrich of Prussia, a cousin of the Kaiser, who strengthened its defences and added many of the romantic towers. And in 1975 the Duchess of Mecklenburg, last descendant of the House of Prussia, sold the castle to opera singer Hermann Hecher, whose family owns it today.
Just looking at the place inspired visions of noble knights and loyal squires, mighty monarchs and beautiful princesses, bloody battles and boozy banquets, and as I daydreamed the mists continued to thin and more fairytale towers appeared on all sides, each with their own stories to tell.
Behind us on a tiny island in the middle of the river was the slim white shape of the Mouse Tower where legend has it the evil Archbishop Hatto of Mainz was eaten by mice.
Just beyond the Rheinstein loomed the shape of another castle, the Reichenstein, razed in 1282 to curb the activities of its robber baron owners, but rebuilt six hundred years later and these days a museum.
And on the opposite bank I could see the lofty towers of the Ehrenfels, another former customs fort, at one time used to store the treasures of Mainz Cathedral, but destroyed after a siege in 1689.
For the next few kilometres it seemed that every bend revealed another castle, and every high point housed a tower, standing protectively over the quaint villages dotted along the banks.
The reason for all those fortifications is that since prehistoric times the river has served both as a hotly disputed border - most notably between Romans and Barbarians, then Germans and French - and a crucial trade route linking most of the countries of Middle Europe with each other and the wider world.
You can, of course, travel through most of this historic landscape by road, but the best and most comfortable way to experience it is surely by boat.
For a start, it's nice to only have to unpack once, have all your meals and accommodation taken care of and be able to focus on the view instead of worrying about staying on the highway. But it also seems very appropriate to journey down the Rhine Valley the way it has been travelled for centuries, and to come upon the castles and villages from the perspective from which they were built, the great river.
My particular voyage was on the Avalon Tapestry, built in the style of a barge and the same basic dimensions as the boats carrying coal and scrap metal up and down the river, 135m long and 12m wide, just able to fit into the many locks along the way.
The Tapestry has two decks of cabins, 84 in total, each with picture windows opening on to the river, so at night I was able to leave the curtains open and watch the passing parade of the river: the working barges with their softly chugging engines; the bright lights of the giant industrial installations which use the Rhine to transport their goods; the sparkling lights of villages and cities; and sometimes, disconcertingly, the stone walls of the locks rising past the window as the water level fell beneath us.
The lounge and dining room are at the bow, offering great views all around, so you can sip a fine Rheingau riesling or a rich, cloudy Hefeweizen beer, dine on metwurst and sauerkraut or apricot-stuffed lamb roast with rosemary gravy, while watching the locals cycling, jogging or walking their dogs along the riverside parks.
There is also a viewing area on the top deck, so on fine days you can lie back in a deckchair and see the spectacular procession of church steeples and cathedral spires, forested slopes and terraced vineyards, gliding past.
And they do seem to glide by because the Tapestry is actually not one boat but two - a small propulsion unit, where all the machinery is housed, pushing from the rear and the floating hotel up front - which means you slide along the river surface with no engine noise and barely a ripple of movement.
To join the Tapestry involved flying into the Swiss financial capital of Zurich and taking a bus to the industrial city of Basel which sits close to the Rhine's source in the Swiss Alps.
From there we spent eight days meandering peacefully for 900km down the Rhine to the Amsterdam-Rhine Canal and the great port of Amsterdam itself.
Along the way we stopped to explore a roll-call of historical cities:
Strasbourg, founded by the Romans as a military outpost, swapped frequently between France and Germany, centre of the Alsace wine district and today the capital of the European Union.
Speyer, a charming, cobblestoned town dominated by a vast cathedral, burial place of eight German kaisers, and inspiration for the building of St Peter's in Rome.
Heidelberg, seat of an ancient university, setting for the musical The Student Prince, watched over by a spectacular red sandstone castle and full of marvellous old taverns.
Mainz, birthplace of Johannes Gutenberg, the father of modern printing, and home of both a majestic cathedral and the superb Gutenberg Museum showing the history of printing.
The beautiful village of Rudesheim, surrounded by vineyards, and full of taverns where you can enjoy the local wine or cafes to savour the amazing Rudesheim coffee made with the local brandy.
Coblenz, where the Moselle River flows into the Rhine, the junction marked by a 50m high statue of Kaiser Wilhelm I on horseback, erected in 1897, torn down in 1945 and rebuilt in 1993.
Cologne, with its vast cathedral that took 600 years to build, described by Unesco as an "exceptional work of human creative genius", which miraculously escaped the World War II bombing which destroyed 90 per cent of the city.
Amsterdam, taking pride in its anarchic approach to life, full of bicycles and cannabis cafes, shop windows offering prostitutes and magic mushrooms, canals and art galleries, delightful markets and fantastic nightlife.
The places we drifted past without stopping, admiring their bridges, spires and domes from afar, also carried names which have resonated down the ages: Mulhausen, Baden-Baden, Kahlsruhe, Mannheim, Worms, Bonn, Dusseldorf, Duisberg, Wesel, Xanten, Arnheim, Utrecht ... the list goes on.
I had planned to do quite a bit of reading on the voyage but that never happened because every bend revealed something fascinating that had fingers pointing and cameras clicking.
Look that huge statue on the top of the ridgeline. It turns out to be a 10m high figure of a teutonic amazon erected by the Iron Chancellor, Otto von Bismark, in 1883 to mark the unification of Germany.
Why are there twin towers on either side of the river? They're the remains of the famous Bridge at Remagen - star of several films and video games - which the German Army failed to destroy, allowing the Allies to get a beachhead across the Rhine in World War II ... though it did later collapse.
What's that huge castle? It's Burg Katz - cat's castle - built in 1370 by the Counts of Katzenelnbogen, blown up by Napoleon in 1806, rebuilt a century later and today a rest home for government employees. And the smaller castle round the next bend is Burg Maus - mouse castle - a name given to it derisively by the Katzenelnbogens which has outlasted its real title.
Is that a knight in armour riding across that plain? Is that really hair hanging down from that pointed tower?
This really is a fairytale landscape. It's the sort of place where you could imagine just about anything.
CHECKLIST
Getting there: You can fly to Europe with Air New Zealand and partner airlines, Swiss and KLM.
Cruising the Rhine: Avalon Waterways offers eight-day Romantic Rhine river cruises, visiting some of Europe's most picturesque regions. The cruises are available from Basel to Amsterdam or from Amsterdam to Basel, visiting the magnificent towns of Strasbourg, Heidelberg, Mainz, Coblenz and Cologne along the way.
Built in 2006, Avalon Tapestry features a lounge and restaurant at its front, as well as an outside viewing deck enabling passengers to fully enjoy the scenery as they cruise. Most staterooms have floor to ceiling sliding glass doors for optimal viewing. All meals onboard are included, with wine complimentary with dinner.
Jim Eagles cruised down the Rhine thanks to Air New Zealand and Avalon Waterways.
