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Xi'an, China (CNN) — For a city with a history of a few dozen centuries, and one that's also home to the 2,000-year-old Terra-cotta Army, Xi'an, China's 637-year-old city wall is a relatively new kid on the block.
Then again, it's a pretty impressive block.
At 12 meters high and 15 meters wide, the 14-kilometer-long Xi'an City Wall is the most complete ancient city wall in China.
It's also the most dramatic place to view one of China's most fascinating cities.
Standing in the city center of Xi'an -- capital city of Shaanxi Province in northwest China -- the ancient structure is the only fortification in China that still stands as an unbroken rectangular circuit.
While it may not get the accolades of that other famed wall -- the Great Wall commonly associated with Badaling near Beijing actually stretches for an estimated 21,000 kilometers -- it's rightly become one of the country's major tourist attractions.
From defense system to city playground
Built in the Ming Dynasty, with intricate drawbridges, towers and a moat, the wall was once one of the most impressive military defense systems in the world.
It safeguarded a city where many Silk Road journeys began and 13 Chinese dynasties had their capital.
Now, the wall stands between modern Xi'an and the old city center.
Outside the wall, highrises and highways reveal new wealth.
Shiny billboards advertise international brands.
Inside the wall, the 36-square-kilometer old Xi'an comprises low, ancient buildings.
No skyscrapers are allowed inside the city wall.
Landmarks such as the Bell and Drum Towers and Muslim Quarter are located within.
Perhaps most interestingly, the top of the wall has become a city playground.
Bike tours atop the wall have become popular with locals and tourists.
One tour starts at the South Gate, the oldest and grandest of the wall's four main gates.
The bumpy, 90-minute ride offers a unique perspective on the two faces of Xi'an.
Zuckerberg's morning run wasn't necessarily novel -- the annual Xi'an City Wall International Marathon is a local highlight.
Since 1993, thousands of athletes from China and overseas gather on the first Sunday in November to compete on a special track.
Declared a major heritage site by the State Administration of Cultural Heritage in the 1980s, the wall has withstood epic tribulation.
Surviving hundreds of years of wars and the vicissitudes of dynasties, the wall was nearly demolished during a social movement in the 1950s (to which historic city walls in places like Beijing and Hangzhou fell victim).
At the time, city walls were seen as a feudal legacy that impeded social advances promised by the Great Leap Forward.
Xi'an's City Wall was saved in part by a renowned historian who opposed the wall's destruction and successfully lobbied then Secretary-General of the State Council Xi Zhongxun, father of current Chinese President Xi Jinping, to spare the wall.
In 1983, it was refurbished as the closed-circuit network that exists now.
It takes about four hours to walk around the wall. Two-hour bike rental costs RMB45 ($7). Entrance fee is RMB54 ($8.50) per person.