After a quiet period during the First World War, a new spa centre designed by architect Paul Wigreux was opened in 1926. In the 1930s, the hotels were occupied not by visitors interested in the waters but by émigrés from Nazi Germany. At the same time it was visited by Polish pianist Arthur Rubinstein. During the Second World War, well-to-do Nazis enjoyed relaxing at "Staatsbad-Mondorf", far away from the bombing and fighting. In 1945, Mondorf's Palace Hotel became "Camp Ashcan", a prisoner-of-war camp for senior Nazi dignitaries who awaited trials at Nuremberg. During the Allied occupation, that lasted until September 1945, there was no permission to transit in the town from 7 pm to 7 am. The spa continued to prosper in the second half of the 20th century with an outdoor swimming pool (1953), a new thermal centre (1988), and the Casino 2000 which opened in 1983.