Slight spoiler
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Well, you can go back to it at any time before joining Oswald for a second journey. There's a recent thread in this forum on methods for beating Battle Squares, which can be found
here.
To gain any aid from your party, you have to keep them out of sight of the golem. The only way to do that is to put them in the north hallway; they can help with ranged attacks and spells, but no melee.