Atop the Drachenfels cliff, high above the River Rhine, sit the striking ruins of a castle. In good weather, thousands of visitors go there to enjoy the panoramic views of the river at Koenigswinter, a bit south of the city of Bonn.

The "Eselweg" (donkey trail), the best-known hiking path up the 321-metre-high cliff, however, has been blocked off for more than a year now due to the extreme danger of rock slides. Now, work is going on to try to secure the rocks. A specialized company has built 50-metre-tall scaffolding on the cliff. On a recent day, Roland Strauss, from the state of North Rhine Westphalia's geological service, was up there and struck the stone with a hammer - and immediately small pieces fell away. "Everything here is crumbly," the geologist says. He then hammers a few times on an old spike in the rock. "Hear that? Hollow. There's no firm hold anymore."

Visitors sit in the shadow of Drachenfels Castle

The spike dates back to the early 1970s when, after a rock slide, the state installed a system of nails and anchors to keep the Drachenfels from breaking apart. The trachyte stone of the erstwhile quarry - which provided stones for, among others, the Cologne cathedral - is porous. "If it hadn't been for this protection system, the upper section including the castle ruins would have fallen long ago," Strauss said. At the end of 2016, measurements were taken that showed there had been some shifting inside the mountain. So the district government based in Cologne shut the Eselweg trail down. Complex renovation work began, an effort that became even more comprehensive as the work continued. "The damage was much greater than originally assumed," says Thomas Metz of the district government. "But that could only be determined little by little.

The Drachenfels (1851), painting by Caspar Nepomuk Johan Scheuren

"One reason is that the cliff has dense vegetation. Only after workers had removed tons of soil, ivy, and plant roots did many cracks become visible, cracks which, in the meantime, have spread through the rock massif. Coloured markings designated the damage areas which are now to be filled with concrete. Last year, two massive rock anchors, 16 and 25 metres long, were driven through the rock in order to prevent a breaking apart of the massif.

At the moment, around 170 spikes that hold the rock together at critical points are being replaced or newly positioned. Before the six-metre-long spikes can be pounded into the rock, large machines must first drill holes for them. Workers are wearing protective masks against swirling clouds of dust. Everything is loud, the scaffolding is shaking. It is still not clear yet when the Eselsweg trail will be opened up again. Most likely in 2019. "Unfortunately everything is taking much longer than planned," Metz says. The originally estimated costs, which the state will bear, are now foreseen tripling to around three million euros (3.5 million dollars). But even after the refurbishing work has been completed, maintaining the Drachenfels will remain a problem. "We cannot prevent the disintegration," Metz says. "We can only slow it down."

Petra Albers (dpa)