During wartime, soldiers applied fire and smoke on beacon towers to pass on military information. During peacetime, the ancient garrison troop engaged in reclamation and grew food.

The Yumen Pass, comprising two fortresses, 20 beacon towers and 18 sections of the Great Wall, is one of the most important passes established by the Han empire at the west end of the Hexi Corridor. More than 2,400 items, including bamboo and wooden slips, silk products, weapons, firewood, farming tools, pottery and lacquerware, among others, have been unearthed there.

In 2014, the Site of Yumen Pass was included on UNESCO's World Heritage List, as part of the "Silk Roads: The Routes Network of Chang'an-Tianshan Corridor", a transnational property of China, Kazakhstan and Kyrgyzstan.

A dozen kilometers away from the ruins of Yumen Pass, to an area beyond the reach of mobile phone signals, lizards disguise themselves by taking on the color of the rocks, bodies unmoving, their tape-like tails roll up and unfold nimbly.

Here lie the remains of a military granary — the other fortress of the Site of Yumen Pass, usually called the Hecang Fortress — which also dates back to the Western Han Dynasty. It's located on the bank of a seasonal wetland, several meters above the riverbed, with ventilation to help tackle humidity.

Ranger Chen Wanying's voice breaks through the sound of the visitors' footsteps. She broadcasts the Notice to Visitors from an obscure bungalow nearby and supervises their behavior, in case they cross the fence, accidentally tread on some relics or unknowingly cause damage.

At this time of the year, surface temperatures can reach nearly 60 C.