If you have the chance, you should go at least once. There are a lot of places in China that I would classify as "you should at least see it once during your lifetime."
One of my favorite was the ancient wall of Xiangyang. I read about about the battle of Xiangyang the history books. However, standing on the massive wall looking across the wide expanse of the Han river, it dawned on me that this fortress stood against 3 Mongol sieges over a period of 16 years and 3 Khanates. If it wasn't for the incompetence of the Song's dynasty central government, the Mongol's conquest of Song would have failed at this wall.