The historic Four Towers building in the town of Bad Ems, in the western German state of Rhineland-Palatinate, was on fire on Wednesday.
The building, which is a UNESCO World Heritage site, caught fire early on Wednesday morning for unknown reasons, according to the district administration.
Almost 150 firefighters were deployed. According to a spokeswoman, the building, known in German as the Haus Vier Türme, is being renovated, which is why no one was in the building in the morning.
The extent of the damage was initially unknown.
Bad Ems Mayor Oliver Krügel has been on site himself since the early hours of the morning, he said. "What is particularly bitter is that the historic roof truss is on fire," he says. "It's lost forever."
The house, also known as Karlsburg, dates back to the 17th century.
"Of course, it is the townscape's defining landmark," Krügel said. "It is also part of the UNESCO World Heritage Great Spa Towns of Europe."
According to the tourism association, the baroque building with four corner towers used to be the unofficial summer residence of Russian Tsar Alexander II.
"It's a bitter pill for the town of Bad Ems," said Krügel. A hotel was supposed to move into the building, which was due to open this year, he added.
The district administration is asking people to avoid the neighbouring area. The firefighting work will continue until the afternoon, said a spokeswoman for the Rhine-Lahn district.
"At the moment, the fire is only being extinguished from the outside, we haven't been able to get a picture of the inside yet."