Prisoner's Gate question

How can I get to prisoner's gate from upper prison? I can't find a entrance in upper prison.

you should find a large Chest, loot that, then around it you should see a staircase? looking path that leads towards the exit to Prisoner's Gate.

I don't know how random the map is there, but I've been through twice and the chest is generally North-North East from where was, and the exit was kind of West/North West from the chest.

Thanks Lycaon :D