The Mongol Empire’s Orkhon Sum complex, a sprawling fortress and palace ensemble built between 1274 and 1405, has been confirmed as the largest castle ever constructed—surpassing even Windsor Castle in size by nearly threefold. New aerial surveys and ground-penetrating radar conducted by the Mongolian Institute of Archaeology and the UNESCO World Heritage Centre reveal the full scale of the site, which spans approximately 1.2 square kilometers (470 acres), compared to Windsor’s 52 acres.
Construction of the complex began under Kublai Khan, the grandson of Genghis Khan, who established it as the political and ceremonial heart of his Yuan Dynasty administration in China. The fortress, later expanded by his successors, served as both a military stronghold and a royal residence, housing up to 10,000 soldiers and administrators during its peak. Unlike European castles built for defense, Orkhon Sum was designed as a khural—a grand assembly hall complex—where the Mongol khans convened with nobles, religious leaders and envoys from across Eurasia.
Historical records from the Yuan Shi, the official dynastic history of the Yuan Dynasty, describe the site as a hub of governance, where decrees were issued and tributary missions were received. The complex included a central palace, administrative buildings, and a vast courtyard capable of accommodating thousands. Archaeologists have identified remnants of a moat, watchtowers, and a system of underground tunnels—features that suggest its dual role as both a ceremonial center and a fortified retreat.
UNESCO, which designated the Orkhon Valley (where the complex is located) as a World Heritage Site in 2004, has emphasized the site’s significance in Mongol history. “This discovery underscores the architectural ambition of the Mongol Empire, which blended military pragmatism with imperial grandeur,” said a spokesperson for the organization. “Unlike castles in Europe, which were often isolated fortresses, Orkhon Sum was a living, breathing center of power—one that reflected the Mongols’ vision of a unified Eurasian administration.”
Excavations over the past decade have uncovered artifacts including bronze seals bearing the imperial insignia, fragments of silk banners, and pottery bearing the mark of the Yuan Dynasty’s kilns. Carbon dating of timber beams from the palace structures confirms their construction between the late 13th and early 15th centuries, aligning with historical accounts of the site’s use until the fall of the Yuan in 1368. The discovery of a previously unknown underground chamber, possibly used for storing state documents, has also fueled speculation about the castle’s role in preserving imperial records.
Mongolian President Ukhnaagiin Khürelsükh, during a visit to the site last month, called the findings “a testament to our ancestors’ engineering prowess and their ability to govern vast territories.” However, preservation challenges remain. The Institute of Archaeology has warned that erosion and urban encroachment near the site threaten its integrity. A $12 million restoration project, funded jointly by Mongolia and the Silk Road Foundation, is underway to stabilize the ruins and prepare them for public access.
The revelation of Orkhon Sum’s scale has also reignited debates among historians about the Mongols’ administrative innovations. While European castles were typically built to dominate local landscapes, the Mongols constructed their complexes to dominate ideological landscapes—serving as physical manifestations of their claim to rule from the Pacific to the Mediterranean. The site’s rediscovery, offers a rare glimpse into how the world’s first truly global empire functioned at its height.
