The builders of Xiadu chose their location with a strategist's eye. The city sits in the Taihang Mountains, flanked by the Beiyi River to the north and the Zhongyi River to the south. This topography made the capital extraordinarily difficult to attack -- any invading army would need to cross water and climb terrain before even reaching the city's massive walls and surrounding moat. The layout was deliberately geometric: a square city divided into eastern and western halves by an internal wall and canal running north to south. The eastern half held the instruments of power -- the palace, the royal cemeteries, and the great workshops -- while the western portion served the broader population. It was urban planning on a scale that would not be matched in many parts of the world for centuries.

What the archaeological record reveals about Xiadu's eastern city is the picture of an industrial powerhouse. Large workshops for casting iron, casting bronze, minting coins, making weapons, making pottery, and working bone have all been excavated there. This was not a ceremonial capital that outsourced its manufacturing. Xiadu made its own tools of commerce and war under the shadow of the palace itself. The iron casting is particularly significant: during the Warring States period, the ability to produce cast iron weapons and tools in quantity was a decisive military and economic advantage. That Yan maintained these facilities at such scale within its capital suggests a state that understood the connection between production capacity and survival.

Two royal cemeteries were discovered within the eastern city, containing 23 tombs in total -- 13 in one cemetery and 10 in the other. Each tomb was covered by a pyramidal tumulus, earthen mounds that would have been visible landmarks rising above the city's roofline. These were the graves of Yan's ruling class, buried within the walls they had built, surrounded by the workshops and palaces that sustained their power. The tombs and their contents were first excavated in 1929 by an archaeological expedition from Peking University, and the discovered objects were eventually housed in a museum built in Beijing. The ruins themselves remain in Yi County, their scale still legible in the landscape even after more than two thousand years of erosion and agricultural encroachment.

Xiadu is the largest excavated city from the Warring States period, yet it occupies virtually no space in Western historical imagination. The era that produced it was one of the most consequential in human history -- seven major states competed for supremacy across what would become China, driving innovations in governance, philosophy, metallurgy, and warfare. Yan was ultimately conquered by the state of Qin, whose first emperor unified China in 221 BC. Xiadu was abandoned, its walls left to settle and crack, its workshops to fill with sediment. But for a hundred years, this city in the mountains between two rivers held a population larger than any other urban center on Earth, a fact that quietly recalibrates how we think about the ancient world.