During its heyday, the Schlossberg’s “Casemate” served a double purpose. By day, it housed stores/goods for the fortress. At night, it became the sleeping quarters for – I assume – very uncomfortable prisoners.
After the Schlossberg was forcibly disassembled in 1807, the Kasematten became an impromptu theater. Beethoven’s opera, Fidelio, was performed here in 1937. You can still attend outdoor productions here.