Flak Tower in a park in Bremen

I spent two days in Bremen in June. It is a nice city not much bigger than Tallinn and it has a picturesque Old Town, too, although not as big and well-preserved, but then again Tallinn was only bombed a couple of times in WW2.
Came across this structure in the largest park in Bremen, marked only as a grey box on the park map, I wonder why ... or maybe I don't. Not explained, but not demolished either, with time taking its toll slowly. If all buildings could age as gracefully as this one :