The old abandoned fortress of Shouchangcheng lies half-buried by sand on the eastern edge of Nanhu, just beyond the trees. Quite why it became abandoned is a mystery as the Nanhu oasis has always been a fertile and well-protected place, with plentiful water.
Curiously, many local people are competely unaware of its existence and the Yangguan Visitor Centre doesn't offer trips there or even provide much information, despite it being a lot more impressive than their lonely little ruined beacon tower.
The desert looks beningn enough - flat and featureless - with little to prevent a short walk to get away from civilization for a while. However, I would urge caution. What looks flat from a distance can hide a lot of dips and depressions that can leave you suddenly out of sight of where you came from. Rises and dips can also quickly hide your vehicle on the road and, voila, you are lost in the Gobi. This is frightening with water and terrifying without. The temperatures drop very fast in the late afternoon and you go from being hot and bothered to miserable and freezing.
So take care of the desert and use a GPS to retrace your steps...and make sure you have spare batteries.