Dr Needleman is in a room upstairs in the Black Boar inn. You can not get the password for his storage area until after you get the battle tower (until then he wears a contraption on his head to block mindreading).
After finishing Maxos temple you get access to sentinel island. There is some stuff to do there before entering the battle tower itself, and until you get most of the way through the tower you can travel back and forth to Broken Valley. There is a warning just before the point of no return.
You will be able to enter the quarry tower in Broken Valley after getting the battle tower (as well as complete a quest on sentinel island that requires flight), but any other open quests in those two areas will fail, and you can not return to most areas.
Your first encounter on sentinel island you can mindread to go up a level, so the higher level you are (I made it to level 18 in the fjords) the more of an experience bonus that will be. You don't need to get to a high level, but it would be better if you got to a new level before entering, to get a full level bonus.
