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×From the point that you have all the yellow doors open (all roaches are on all the pressure plates, you are on the X), step west into the shallow water. The roaches won't see you, and so the doors will stay open, but the guard will, so he will move up as far as he can go (go up to the top of the shallow water so that he is as far north as you can get him. From there, become visible again by stepping back on the X.
Let the roaches go down as far as they will all go, and forget about the pressure plates now. Then go and stand on the trapdoor at the top.
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