Winged 557 miles southwest from Beijing to Xian, which as you no doubt know was the eastern terminus of the Silk Road and during the Tang Dynasty probably the largest and most developed city in the world. It is still now probably the last large walled city in the world. The wall surrounding the inner city is a total of 7.3 miles long, forty-nine feet high, and fifty-nine feet wide at the top.
The 7.3 mile-long top of the wall provides a nice walking and biking path. Notice the bicycle-built-for-two.