Flakturm

This is one of two Flak Towers (I recommend reading this article) located in the Augarten. The other is rectangle in shape.  As explained in the article flak is a German acronym meaning anti aircraft guns. 

 Built by the Germans in 1942 as anti aircraft defense they could also house many thousands of troops. What I found interesting is that the explosives required to destroy these structures would also destroy the surrounding neighbourhood.

There is a total of  four other  flakturms still standing in other parts of Vienna.