If I remember correctly, you have to go to the east part of the area. Go down a hall, and there at the end, you'll find a very small whole in the wall where you can go upwards. It's very difficult to find, but it is there.
@John1: The treehouse is in the north east section of the forrest. It's quite difficult to find. Track one of the trees, climb up, be careful they are very high, and then check if you found the right tree which leads to the treehouse. There are four trees up to the treehouse if I remember correctly, but be careful, they can be cut down. If you try to get down from the treehouse when the tree is cut, you'll die.
What's up there? Well, that's up to you to discover. I tell you, it's cool up there and it is worth it.
Best regards SLJ.
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