While I can't say exactly where the Forbidden City photo of Peter/Sarah was taken, it probably helps that we can eliminate one of the four gates that lead into the palace (there's one for each cardinal point).
The East Opulent Gate (东华门) is slightly different from the rest, as due to superstition and geomancy and stuff, only eight columns of doornails were installed on each side. Now if Peter and Sarah (the usual nine columns on their door) aren't at the East Gate, and their shadows point straight into the Forbidden City, it means that the photo wasn't taken in the morning. Consequently, we can deduce that there isn't Hours of Operation bunching at the Forbidden City, and depending on the arrival times of their flights, we would know about how far into the leg the location is.
Instinct tells me that it's the South Gate (Meridian Gate), since Beijing is after all high in the Northern Hemisphere. However, the total lack of other tourists confuses me quite a bit. Any ideas?
Since the Great Wall is indeed on the way to Mongolia (and they drive there), could this be one long leg instead of two separate ones? And there typically (well, actually, always) isn't a Roadblock on the first leg, what could this mean for the scaling of the wall?
Also, I have something for Vipul and Arti in that photo... They look good...