The Oubliette: Solution
She closed her eyes and hummed some sort of tune, while swiftly making elaborate gestures with her hands. A piercing outline of light appeared on the dark wall in the shape of a door. The darkness within that outline dissolved away.
“How did you do that?” said Laura.
“Well, that’s easy,” said Jenny. “My counterpart in this world lived at around the time this complex was built.. She bribed the architect to build in an escape route. Like so.” She looked off into the distance, apparently at no one. "And none of you gave any suggestions. I’m disappointed in you.’ She stuck out her tongue.
“Er, Jenny, who are you talking to?”