Before Qin unified China into a centralized empire in 221 B.C, China was split into 7 kingdoms. Qin's kingdom grew to be the strongest and all the kingdoms continued to fight until 221 B.C when Qin's kingdom defeated all the others. The unification not only ended 6 centuries of war it also started a centralized imperial system which was to last for over 2,000 years.
Qin standardized money, weights and also simplified Chinese writing down to 9,000 simple symbols, which are still being used today. Chinese money was a gold or bronze coin with a small square hole in the centre. Standardizing weights helped to prevent corruption and helped to encourage trade because people could trust the measurements being used.
Qin was a very harsh ruler. When coming to the throne, Qin burnt hundreds of history books and executed hundreds of scholars to try and "erase history". He wanted his citizens to forget the past kings and focus on the now unified China.
Qin sent peasents, political exiles and anyone who disagreed with him to help build the great wall. Instead of punishing people for crimes he would send them to work on the wall, which was basically the death penalty. Qin even sent his own brother to work on the wall as punishment after questioning him about his harsh rule.