The Adventurers' Guild

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Yes, the dungeon you missed would help you to get in the forest (avoiding the combat with the goblin guards). The idea was that the gurads are too strong for the characters in lower level. So they have to find another way to get in the forest.

"One of the four is on this level" is maybe a part of a riddle. I don't konw...

Well, if you're going to be deliberately obtuse, I'll just give up then.