On the opposite side of the Rhine from Coblenz, and towering above Lahnstein, rises Castle Lahneck, a keep shaped somewhat in the form of a pentagon. Lahneck succumbed to the hordes of Louis XIII. in the same year as the castle of Heidelberg was destroyed. The following stirring tale is associated with Lahneck.

It was the Templars of Jerusalem who erected this fortress whose imposing watch-tower rises nearly 100 feet above the main building. The riches of the Templars led to their destruction. The contemptible French king, Philip the Fair, by making grave complaints to the Pope obtained an order for the abolition of this much-abused order, and dragged the Grand Master with fifty of his faithful followers to the stake. Everywhere a cruel policy of extermination was immediately adopted against the outlawed knights, the chief motive of the persecutors being rather a desire to confiscate the rich possessions of the Templars than any religious zeal against heretics and sinners.

Peter von Aspelt, Archbishop of Mainz, had cast envious eyes on proud Lahneck which sheltered twelve Knights-Templars and their retainers. Alleging some faulty conduct on the part of the soldiers of the cross, he gave orders that the castle should be razed, and that the knights should exchange the white mantle with the red cross for the monk's cowl, but to this the twelve as knights sans peur et sans reproche issued a stout defiance. This excited the greed and rage of the archbishop all the more. From the pontiff, whom with his own hands he had successfully nursed on his sick-bed at Avignon, Peter von Aspelt procured full power over the goods and lives of the excommunicated knights of Lahneck. He then proceeded down the Rhine with many vassals and mercenaries, and presented the Pope's letter to the Templars, at the same time commanding them to yield. Otherwise their castle would be taken by storm, and the inmates as impenitent sinners would die a shameful death on the gallows. The oldest of the twelve, a man with silvery hair, advanced and declared in the name of his brethren, that they were resolved to fight to the last drop of their blood, and further, that they were quite prepared to suffer like their brethren in France. And so the fight between such fearful odds began. Many soldiers of the Electorate fell under the swords of the knights and their faithful servants, but ever the furious archbishop ordered forward new bands to fill the gaps. Day by day the ranks of the defenders became thinner. Prominent everywhere in this hand to hand struggle were the heroic forms of the twelve Templars, in white mantle with blood-red cross. At last, at a breach which had been defended with leonine courage, one of the noble twelve sank beneath his shattered shield, and closed his eyes in death. A second shared his fate, then a third. The others, bleeding from many wounds and aided by the sorely diminished remnant of their retainers, redoubled their brave efforts, but still death made havoc in their ranks. When, on the evening of the day of fiercest onslaught the victorious besiegers planted their banner on the captured battlement, the silver-haired veteran, the former spokesman, stood with blood-flecked sword among the bodies of his fallen comrades, the last survivor. Touched by such noble heroism the archbishop informed him that he would be allowed to surrender; but calling down the curse of heaven on worldly churchmen and their greed of land, he raised on high his sword and rushed upon his foes. Pierced with many wounds the last of the twelve sank to the earth, and over the corpse of this noble man the soldiers of Mainz pressed into the fortress itself.

Peter von Aspelt preserved Lahneck as a place of defence and residence for an officer of the Electorate of Mainz, and nominated as first holder of the post, Hartwin von Winningen. The castle remained in the possession of the Electorate of Mainz for 300 years, but the sad story of the twelve heroic Templars is remembered in the neighbourhood of Lahneck to this day.