I just finished reading the interesting book
The Zookeepers' War: An Incredible True Story from the Cold War (Nov. 2019 edition) and there is one page that briefly discusses Krefeld Zoo's 'new' ape house. "But just one year after the all-weather zoo (Munster) opened its gates, the nearby Krefeld Zoo set a new standard in keeping apes. There, gorillas, chimpanzees, and orangutans lived in family units in a tropical hall with a warm, humid climate and jungle plants, separated from visitors by only a narrow moat. Krefeld's tropical ape house was a pioneering structure in modern animal keeping..."
It's fascinating that what was once exemplary is now considered atrocious. I know that in North American zoos at least 95% of all great ape exhibits have an attached outdoor enclosure and I'm guessing that the same applies to European zoos. Hopefully Krefeld can raise enough funds to build a world-class Primate House in the near future.
Former Chimpanzee exhibit (with no outdoor access) at Krefeld Zoo:
Former Orangutan exhibit (with no outdoor access) at Krefeld Zoo: