【正文】 defensive fortification back to the year 656 . during the reign of King Cheng of the States of Chu. Its construction continued throughout the Warring States period in the fifth Century . when ducal states Yan, Zhao, Wei, and Qin were frequently plundered by the nomadic peoples living north of the Yinshan and Yanshan mountain ranges. Walls, then, were built separately by these ducal states to ward off such harassments.　　Later in 221 ., when Qin conquered the other states and unified China, Emperor Qinshihuang ordered the connection of these individual walls and further extensions to form the basis of the present great wall. As a matter of fact, a separate outer wall was constructed north of the Yinshan range in the Han Dynasty(206 BC1644 BC.), which went to ruin through years of neglect. In the many intervening centuries, succeeding dynasties rebuilt parts of the Wall. The most extensive reinforcements and renovations were carried out in the Ming Dynasty (13681644) when altogether 18 lengthy stretches were reinforced with bricks and rocks. it is mostly the Ming Dynasty Wall that visitors see today. The Great Wall is divided into two sections, the east and west, with Shanxi Province as the dividing line. The west part is a rammed earth construction, about meters high on average. In the eastern part, the core of the Wall is rammed earth as well, but the outer shell is reinforced with bricks and rocks. The most imposing and best preserved sections of the Great Wall are at Badaling and Mutianyu, not far from Beijing and both are open to visitors. The Wall of those sections is meters high and meters wide at its base, narrowing to meters on the ramparts, wide enough for five horses to gallop abreast. There are ramparts, embrasures, peepholes and apertures for archers on the top, besides gutters with gargoyles to drain rainwater off the parapet walk. Twostoried watchtowers are built at approximately 400meters internals. The top stories of the watchtower were designed for observing enemy movements, while the first was used for storing grain, fodder, military equipment and gunpowder as well as for quartering garrison soldiers. The highest watchtower at Badaling standing on a hilltop, is reached only after a steep climb, like climbing a ladder to heaven. The view from the top is rewarding, hoverer. The Wall follows the contour of mountains that rise one behind the other until they finally fade and merge with distant haze. A signal system formerly existed that served to municate 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 munications. 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. (16441911) Jiayuguan Pass was not so much as the Strategic pass Under the Heaven as an important m