Cologne really is a great zoo, with many unique species, such as litlle blue penguin and douc langur. And as you can see, the elephant house is just great. The photo shows about half the space of their indoor stable, and the also have a large basin filled all the time, so they always have access to water. These elephants can easily spend their winters indoors in Cologne, even though they also have a large outdoo area.
From the photos and descriptions, this facility seems far superior to the much-hyped Copenhagen elephant exhibit, which seem barren and small by comparison.
I didn't realize Cologne did not have soft substrate indoors--they definitely loose points for that. Copenhagen's open roof structure is also very good.