Treehouses Achievement in The Witness
Activate the Treehouse laser.
How to unlock Treehouses
The Treehouse area is located between the Keep and the Swamp. It is initially only accessible by activating one of the many boat locations around the perimeter of the island and then sailing to the back entrance of the Treehouse area. Once you work your way through the Treehouse puzzles, you'll be able to drop a bridge leading to the Keep, allowing you to access the Treehouse area from land.
The Treehouse is the first area to introduce you to Suns in the puzzles, so this is where you'll find your Sun tutorial. In addition to Suns, the Treehouse puzzles also use black/white squares, tetris shapes, and multi-colored squares and sun. So you'll want to make sure you've done those tutorials first.
For a walkthrough of the Treehouse area, use THIS LINK for the puzzle solutions.
Once you've solved all the puzzles and created the walkways, you'll now need to activate a panel on the center tree, which opens a nearby door for a short amount of time. The only way to make it to the door in time is to solve the two orange walkways such that they overlap, making a shortcut between the two trees.
Once you get inside the treehouse, activate the laser panel and the achievement will unlock.