A signal system formerly existed that served to communicate military information to the dynastic capital. This consisted of beacon towers on the Wall itself and on mountain tops within sight of the Wall. At the approach of enemy troops, smoke signals gave the alarm from the beacon towers in the daytime and bonfire did this at night. Emergency signals could be relayed to the capital from distant places within a few hour long before the invention of anything like modern communications.

    There stand 14 major passes (Guan, in Chinese) at places of strategic importance along the Great Wall, the most important being Shanghaiguan and Jiayuguan. Yet the most impressive one is Juyongguan, about 50 kilometers northwest of Beijing.

    Known as "Tian Xia Di YI Guan" (The First Pass Under Heaven), Shanghaiguan Pass is situated between two sheer cliffs forming a neck connecting north China with the northeast. It had been, therefore, a key junction contested by all strategists and many famous battles were fought here. It was the gate of Shanghaiguan that the Ming general Wu Sangui opened to the Manchu army to suppress the peasant rebellion led by Li Zicheng and so surrendered the whole Ming empire to the Manchus, leading to the foundation of the Qing Dynasty. (1644-1911)

    Jiayuguan Pass was not so much as the "Strategic pass Under the Heaven" as an important communication center in Chinese history. Cleft between the snow-capped Qilian Mountains and the rolling Mazong Mountains, it was on the ancient Silk Road. Zhang Qian, the first envoy of Emperor Wu Di of the Western Han dynasty (206 B.C-24 A.D), crossed it on his journey to the western regions. Later, silk flowed to the west through this pass too. The gate-tower of Jiayuguan is an attractive building of excellent workmanship. It has an inner city and an outer city, the former square in shape and surrounded by a wall 11.7 meters high and 730 meters in circumference. It has two gates, an eastern one and a western one. On each gate sits a tower facing each other. the four corners of the wall are occupied by four watch towers, one for each.

    Juyongguan, a gateway to ancient Beijing from Inner Mongolia, was built in a 15-kilometer long ravine flanked by mountains. The cavalrymen of Genghis Khan swept through it in the 13th century. At the center of the pass is a white marble platform named the Cloud terrace, which was called the Crossing-Street Dagoba, since its narrow arch spanned the main street of the pass and on the top of the terrace there used to be three stone dagobas, built in the Yuan Daynasty(1206-1368). At the bottom of the terrace is a half-octagonal arch gateway, interesting for its wealth of detail: it is decorated with splendid images of Buddha and four celestial guardians carved on the walls. The vividness of their expressions is matched by the exquisite workmanship. such grandiose relics works, with several stones pieced together, are rarely seen in ancient Chinese carving. The gate jambs bear a multi-lingual Buddhist sutra, carved some 600 years ago in Sanskrit(3), Tibetan, Mongolian, Uigur(4), Han Chinese and the language of Western Xia. Undoubtedly, they are valuable to the study of Buddhism and ancient languages. ()