These were taken in a December 2005 visit to Berlin, please excuse the poor picture quality as i'd forgotten my DSLR. As you probably know Humboldthain is the only remaining flak tower in Berlin, the Berlin zoo tower was demolished (with great difficulty) in the late 40s early 50s, as was the Friedrichshain tower which i think there parts of one wall still standing.
The Humboldthain tower was part buried in rubble from all the bomb damaged buildings in the area, the hill you see is not natural. If you visit from March to October you can tour the interior of the tower with Berliner unterwelten http://berliner-unterwelten.de/ .It's closed during the winter because bats hibernate in it.
I was amazed at the amount of artillery hits it had taken, i read the Russkies had their biggest guns firing virtually point blank and the shells just bounced off.