Zisterne - Cistern on Grazer Schlossberg with ornamental wrought iron top

Grosse Zisterne

Great Cistern 

Built to serve as a water-collection and storage facility in the event of sieges by invading armies. Unlike the Tuerkenbrunnen, this is not a well, but essentially a tank for capture and retention of rain water. The Turkish Well, by contrast was dug all they way down through the mountain to provide access to ground water beneath the city.