The Wien is usually a ridiculously tiny river. But due to surrounding geology, heavy rain can sometimes make it grow to 2000 times its size. These massive basins can hold the water for a few hours to protect the city.

Since the basins are only rarely filled, a lot of plant and animal life develops in them, and the huge walls are a beautiful contrast to that. You are not supposed to walk on the walls, but you can. The spot that I put the pin in is nice for picnics.