Ruins of Kostrzyn Castle

The castle is one of the most important buildings of the Kostrzyn Fortress, inextricably linked with it. Erected by the Teutonic Knights, after reconstruction it was the seat of John Hohenzollern. In the 17th and 18th centuries it housed a prison where, among others, The heir to the Prussian throne, Frederick, stayed in 1730–1731. After a fire in 1758, it was rebuilt and turned into a barracks in 1814. In 1945, it was burned and partially damaged, but most of the walls remained. In 1969, the surviving fragments were blown up and demolished. The remains of walls, stairs and cellars have survived to this day, but some of them have already collapsed.

Huge area, today hardly recognizable as a town or fortress in its entirety.
Lots of information boards.
More information: de.m.wikipedia.org/wiki/Festung_K%C3%BCstrin

These cavities in the old ruins are full of mystery.