The reconstructed section of the great wall at Jiumenkou is quite small, and also features a rather sorry bird zoo. Rather than paying for the ticket and visit the actual site, I’d much rather recommend to climb up to the right along the unrestored section: good views, much less crowded. You don’t even need to buy the entrance ticket to do so (and you cannot cross from the official site to the unrestored section): At the end of the parking lot, instead of walking left and entering via the “normal” tourist ticket office/entrance, walk right and cross the river. Turn left and walk towards the wall for some 100m, then take the stairs to the right that start from an unpaved “parking lot” (the first 10m or so of the stairs are missing). The stairs lead up to a well maintained Chinese-style stone paved path up to the unrestored towers and section of the wall. Along the path, there is a mine – if you bring a flashlight (and are prepared for wet feet), you can go explore.
Also note that Jiaoshan (Great Wall at Shanhaiguan) is closed for the time being (fenced off, and with guards) – if you go to Shanhaiguan to visit the Great Wall, the taxi drivers will (have to) take you to Jiumenkou.
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