Stone structure, maybe an old fortifikation. Has been sealed with concrete.

via StreetComplete 53.3

Attached photo(s):
https://westnordost.de/p/171938.jpg
https://westnordost.de/p/171939.jpg
https://westnordost.de/p/171940.jpg
https://westnordost.de/p/171941.jpg
https://westnordost.de/p/171942.jpg