The puzzle is harder to solve than to find.
Map
Treehouse Obelisk
West 1
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It is inside the castle (aka the "Keep")
You need to be high up to solve it.
Look carefully at the start and end points of these symbols, then look down at the giant floor panel mazes in the castle.
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(The 4th and 5th symbol both match the same floor panel maze.)
This 1st symbol matches the yellow floor panel maze in the castle (the bottom right, when viewed from above).
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You will need to trace a solution path you've entered on the floor panel maze.
The yellow puzzle is fairly straightforward; you just need a path whose view isn't blocked by statues or other debris. You should be able to work it out easily enough.
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Complete spoiler:
This is a really good place to stop reading. You now know roughly where it is.
Roll over each black area below for hints.
Each new hint gives away more than the previous one.
Note that all the symbols on this face look like grids.
Each of the symbols on this face match one of the floor panel mazes.
Intro
This one should provide a fair challenge.
In fact, all of the puzzles on this face require some clever puzzle solving.
What's that? It's blocked by some obstacle? Try finding a different solution path.
Show hints
THE WITNESS - ENVIRONMENTAL PUZZLE HINT GUIDE