Oberwesel , on the left bank of the Rhine and just to the north of Kaub, possesses the finest extant fortifications in the Rhineland, a three-kilometre-long Stadtmauer which still preserves sixteen of its towers from an original total of twenty-one. Unfortunately, the section guarding the riverfront is marred by the proximity of the railway, which runs directly alongside, but the whole ensemble is nonetheless impressive, not least at the hilly northwestern edge, where the fortified church of St Martin (popularly known, for obvious reasons, as the "white church") forms an integral part of the municipal defences. At the opposite end of town is a huge fortress, the Schönburg , which is now a ruin but still impressive. Below it stands the Gothic Liebfrauenkirche (which the locals call the "red church", after the huge red sandstone blocks used to build it). In its airy interior, look out for the St Nicholas altar, on which the saint is shown helping three Knights who have been sentenced to death for crimes they didn't commit, saving three sisters who have been forced into prostitution by their father and protecting passengers on board a ship.