So, go back into the room you just left?
No, the door previously guarded.
Logic implies that by distance to the door to backstage it leads to the concert hall, and despite my digging through the thread a bit I was unable to find the reason why we wanted to go to the concert hall amongst all the stuff. I just remember us wanting to do that. For some reason other than shoving grenades down freaky spider person throats.
Edit: This is why you need your own webspace of some form to post this stuff, gog damn.