For people who cannot continue after clue 4, enter the backstreet and speak with Subong, a single worrior, then he tells you to go through the door behind him - enter the backstreet stonehall and the mystery quest will be unlocked and you can continue :)
got in but no cage? then you want to go back outside but gate is closed? use unstuck then you will be teleported close to the portal go out speak to Subong then go in again this is based on my experience