However the title suggests to me it was a flak fortress or anti aircraft gun atop a concrete bunker from WWII. Which to my way of thinking is a modern version of a castle, which was an older style of fortress.
I'm assuming (once again) that this may be either on Fyn or Sjaelland, I visited Fyn a few times in another life and am pretty sure I remember seeing something like this.
Apart from all that, it's a striking image, the film combined with the filter, has made this into what it is.