You're in the right place. But the door is locked and trapped and cant be forced it sounds like you are going to have to boot one of your characters out, import or find a thief with high find traps and pick locks skills and then try it-Is Nalia still alive-go ger her, soup her up with potions of mastery thievery and perception and you should be able to get in the door. Look out for the trap though it'll kill you if you fail your save.
Alternatively try casting knock on it then having one of your expendible characters opening it (I'd save it first) I've not tried that it might work.
hope that helps mate, let me know how you get on-getting in first, then in the fight
