The Great Wall of China is a series of fortifications built across the historical northern borders of ancient Chinese states and Imperial China as protection against various nomadic groups. Construction began as early as the 7th century BC, with the most famous sections built by the Ming Dynasty (1368–1644). Contrary to popular myth, the wall is not one continuous line but a collection of walls, trenches, and natural barriers like hills and rivers, stretching over 21,196 kilometers. A unique ingredient used in the Ming-era mortar was sticky rice flour, which created a chemical bond with the calcium carbonate, making the wall incredibly durable and resistant to earthquakes. It served not only as a defense but also as a means of border control, allowing for the imposition of duties on goods transported along the Silk Road and the regulation of immigration and emigration.