During the end of the Seven Warring States (战国七雄) era, when the Qin Kingdom (秦国) unified the other states, Shi Huang Di (秦始皇) also unified the writing system, declaring only the Qin's writing system, which is known as Small Seal Script (小篆) is allowed, and the rest is abolished.
However, China has at least been conquered by two civilization that has their own writing system, which is the Mongols (蒙古) and the Manchus (满族). Didn't the Mongols and Manchus replace their own scripts with the Chinese characters?
How did that Chinese character remains readable till date?
It would not have been practical for the Mongols and the Manchus to replace Chinese characters with their own scripts. There were more Chinese than there were Mongols and Manchus, and at the end of the day, they still depended on the the Chinese to help them rule the empire. It was much more easier and convenient to simply have their own people learn Chinese characters. The Manchus did keep their own written script - every government document that was written in Chinese was also copied in Manchu. You can see this in Beijing today - signs from the Qing in the Forbidden City have Chinese characters on the right and Manchu script on the left.