And for the another way of getting inside: Has anyone of you tried the ressurection scroll or spell? Just let one character die on the impenetrable wall as close to the stairs as you can and revive him afterwards. The revive range varies, but the Teleport after revive is 10 meters set - you can easily teleport him up the stairs. It worked well for me and probably the easiest way which costs just one ressurect scroll.
