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The oldest Chinese building ---the original Great Wall (2200 years old) was a mud wall, not even a mud brick wall. The Great Wall you see today was built after the Mongol Invasion.

Comments  

The Wall is not a building. The dash is unnecessary and therefore wrong. You need commas in the larger numbers. The parentheses are distracting. One idea:

"The oldest extant Chinese construction, the 2,200-year-old Great Wall was originally built of mud, and not even mud brick. The Great Wall you see today was built after the Mongol Invasion."

I'm not perfectly happy with using the parenthetical as an introduction because it seems to imply that its being the oldest is why it was mud, which may be true but is beside the point. Maybe:

"The 2,200-year-old Great Wall, the oldest extant Chinese construction, was originally built of mud, and not even mud brick. The Great Wall you see today was built after the Mongol Invasion."