The exact location of the fabled Yangguan is not actually known, something that is not explained until you arrive at the Yangguan Visitor Centre. At Yangguan there was the curio beach, where over centuries, locals dug up artefacts from the desert sands, and this is the area where the Visitor centre now lies.
Various historical records vary in where they locate Yangguan, but one theory is that it was at Gaotai by the small river that is just before you reach Nanhu and Yangguan. This would make some sense as the bigger fortresses and passes in this area were all located near running water. Also, the present location of "Yangguan" would seem to be too close to the old town of Shouchangcheng (see Nanhu).
Wherever it lay, the small gorge of the Gaotai - maybe 10 metres deep - is a pleasant place to explore, and there is always the possibility that you will be the one to turn a corner by the quiet waters and discover a long-lost city bustling with merchants, monks and envoys...
[Photos to follow]
The Visitor Centre is unusual because it is privately owned and is very, very good. It has not one but two museums. One is on the 'end of the Great Wall' and is an excellent presnetation, in English and Chinese, on the history and archaeology of the area. It has great descriptions and uses a number of scale models to show it all. This is the museum building on the left. The opposite building contains a superb exhibition on the Silk Route, one of the few (perhaps the only?) museum in China to consider anything beyond the administrative borders of the area. This museum puts the local area in a wider context and helps to bring the whole Jiayuguan-Dunhuang-Yangguan experience to life. These two museums should be visited *before* visiting the Mogaokou near Dunhuang!
The Visitor Centre has a lot that will entertain and educate the whole family. Although the fortress is probably a bit unrepresentative of the original Yangguan fortress, it is done in a way that seems acceptable (and it is far enough away from the beacon tower to be unobtrusive).
At the back of the complex is a recreation of the pass gateway, and visitors are first issued with a passport printed onto bamboo slips (it helps if you already have a Chinese name). This is then presented to the fierce looking guard at the gate who stamps it before letting you "leave" China. Kids would love it. I loved it. For once, the dressing up and role playing seemed both appropriate for the situation.
To one side of the fortress is a Han military camp which again adds to the feel of the place. None of it is authentic, but then the whole thing is presented as a recreation. If you want to skip these, it is easy to do so and just wander up to the beacon tower and gaze out at the cruel desert beyond.