What iconic fortification was built in the late 11th century along the northern border of Korea?

Literally translating to the "Thousand Li Wall," Cheolli Jangseong is an 11th-century structure built for northern defense. Located in what is now Northern China, the fortification ran from Buyeoseong to Bohai Bay for 1000 li (which is a "Chinese mile," or about one-third of a regular mile).

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