Look in the starting area

You may have stepped near or even on top of it

Look down into the starting area

Get up on the wall surrounding the starting area. (To get on the wall, exit the main gate of the starting area and hug the left side of the path. You should find a hill leading back towards the wall. Enter the walkway where the side of the wall is crumbling.)

Towards the back, you can walk up on top of the overhang that covers the tunnel you emerged from at the start of the game. Look over the right edge at a small garden below. The rectangular border of this garden contains the puzzle with the starting point in the center of the side closest to you.

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It is very likely you have come quite close to this puzzle once before

You won't be able to light this one up at ground level