After defeating Amdusias (and getting the book off the alter) one of the corridors there that was blocked becomes open (opposite the chamber with the bedrooms upstairs). You may want to finish off Broken Valley first, if you have not done so already.
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.
Welcome to the forum.
