Fortifications have stood at this choke point since at least the Northern Qi dynasty in the 6th century. The Sui and Tang dynasties built their own defensive works here, and by 785 a permanent garrison called Yuguan occupied the site. The Liao dynasty, founded by the Khitan people who were themselves the kind of northeastern invaders the pass was designed to stop, controlled the area for two centuries. But the fortification that stands today dates to 1381, when Ming general Xu Da was ordered to repair the old passes of Yongping and Jieling. His engineers looked at the terrain -- mountains to the north, sea to the south, a river on the east bank -- and built something far more ambitious than a repair. They named it Shanhaiguan: the pass between mountains and sea. Two centuries later, General Qi Jiguang transformed it into a full military city, adding forts and walled enclosures to the east, south, and north until it became one of the most heavily fortified positions in China.

The pass earned its famous inscription -- 'Greatest Pass Under Heaven' -- because geography left invading armies no good alternatives. The Liaoxi Corridor narrows here to a strip of coastal plain barely wide enough for an army to march through. Without taking Shanhai Pass, an invader from the northeast had only one option: circumvent it by marching deep into the Yan Mountains through treacherously narrow passes that made it nearly impossible to maintain supply lines. Large armies could not sustain themselves on such a route. The Dongyi, Donghu, Xianbei, Wuhuan, Khitan, and Jurchen all pressed against this bottleneck across the centuries. When the Yongle Emperor moved the Ming capital from Nanjing to Beijing in the early 15th century, Shanhai Pass became the single most important defensive position in the empire, shielding the heartland around the capital from everything that lay beyond.

The Great Wall does not simply end at Shanhai Pass. It wades into the sea. The section known as Old Dragon's Head -- Laolongtou -- extends the wall's masonry into the waters of the Bohai Sea, as if the builders intended to fortify the shoreline itself against amphibious assault. The pass proper is built as a square with a perimeter of four kilometers. Its walls rise 14 meters high and measure 7 meters thick. Deep, wide moats surround three sides, crossed only by drawbridges. A bell tower stands at the center. Of the four original gates, only the Zhendong Gate on the east wall survives -- the most important of the four, because it faced outward, toward the territory the pass was built to watch. The gate's famous plaque bearing the calligraphy 'Greatest Pass Under Heaven' has become one of the most photographed inscriptions in China, a UNESCO World Heritage Site since 1987.

The pass changed hands repeatedly through the modern era, each transition reflecting a larger upheaval. In July 1900, 15,000 Japanese troops landed here as part of the Eight-Nation Alliance marching to relieve the Siege of the International Legations in Beijing during the Boxer Rebellion. A drunken brawl between Japanese and French soldiers at the pass killed ten men -- a minor incident that hinted at the fragility of the alliance. During the Republic of China, Shanhai Pass was controlled successively by Zhang Zuolin's Fengtian clique, Chiang Kai-shek's Nationalists, the Imperial Japanese Army, and finally the Communist Northeast Field Army, which took the pass on November 27, 1948. Each new occupier understood what the Ming generals had understood five centuries earlier: this strip of land between mountain and sea is the lock on the door between two halves of a continent. Lose it, and the door swings open.