Seems like a bunker

We still have a lot of those in Germany, but I don't know this particular one. They were built during the later stages of WWII. They're very difficult to tear down but also not much use for anything so they usually just sit there, looking ugly.

Pallasstraße, Berlin

https://en.wikipedia.org/wiki/Air_raid_shelter#Hochbunker

Note the lack of windows, tiny door, and extreme wall thickness.