I had the same problem. I couldn't a switch or trigger for the poison. Before I completed the battle tower quest I went back (lvl 19) and made it through. There are places you can stand where you don't get poisoned. Try going the left side and waiting on top of the stairs. All you really need to do is rush in and kill the guy who greats you for the door to open into the next room. I thought the bottle of rum quest item you find outside might have been part of the quest but it isn't. There is also a book before you go in which says if you offer a unique creature you will be granted access, but I don't know where you to find it, I had heaps of body parts but nothing. Good luck and post if you find a trigger or switch for the poison.
