Thanks for the pbase link, puddin. After looking at hundreds of Forbidden City pix, I think I found a possible candidate for "our" door. It's taken from an opposite view, looking out, but the floor tiles are a good match, the hinge looks about right, and if you look closely, you can see that the inner red walls are covered with smaller "knobs", just like the ones above Peter's head (highlighted with a yellow circle). If it isn't the one, it's very close.
If we can identify the building in the background, we may get closer to identifying this structure.
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