Flak towers (Flaktürme) were eight complexes of large, above-ground, anti-aircraft gun blockhouse towers constructed by Nazi Germany in the cities of Berlin, Hamburg, and Vienna from 1940 onwards. The towers were used by the Luftwaffe to defend against Allied air raids against these cities during World War II. They also served as air-raid shelters for tens of thousands of local civilians.
In Vienna military authorities chose the Augarten as one of several places to erect massive buildings for anti-aircraft defence to protect the inner city from Allied bombing. During summer 1944 the construction of a 55 metre high tower with platforms for anti-aircraft guns and nearby also a 51 metre high control tower was begun but not finished. Their remains are still visible in the middle of the park. Moreover during the war hundreds of cubic metres of rubbish were dumped on the site whilst armoured vehicles criss-crossed the garden and - as it is supposed - common graves were dug for hundreds of war victims.
References:House of the Blackheads (Melngalvju nams) is a building situated in the old town of Riga. The original building was erected during the first third of the 14th century for the Brotherhood of Blackheads, a guild for unmarried German merchants in Riga. Major works were done in the years 1580 and 1886, adding most of the ornaments.
The structure was bombed to a ruin by the Germans June 28, 1941 and the remains demolished by the Soviets in 1948. The current reconstruction was erected from 1995 to 1999. Today the House of Blackheads serves as a museum and sometimes concert hall.