Read my other posts. Qin was one of the most aggressive militaristic nations in the entire history of China - the economy, the culture - everything was geared towards war and especially conquest. And Qin Shi Huang-di and his 9 year steam-roll of the rest of China which killed 2 million people was the embodiment of this culture. Even before the steam-roll, Qin was well-known as one of the biggest bully boys - forcibly annexing numerous smaller kingdoms around it. In military terms, Qin were the Nazis of ancient China. Robot-like ruthless merciless efficiency and will to conquer. He was also one of the most organized leaders in Chinese history. This organization and centralization of power and his conquest of China through military might were his two most powerful legacies to history.
Building the Great Wall is hardly his most important legacy to China, just the one with the best picture you can stick on a postcard. It was quite literally a side-note to his "career". BTW, practically *every single* Chinese leader to ever exist had a "protective" attitude to the northern barbarians. It was hardly the focus of his career. More like an appendix. If you're talking about any of the Song dynasty emperors though...You might as well give "protective" to every Chinese emperor in existence if you're going to use that definition of "protective". The northern barbarians to China is like arguing over the role of religion in America. It's the continual thorn on the side that comes up all the time, the problem that never ends. Even if he never built the Great Wall he would still be remembered as one of the most famous and influential Chinese leaders in istory. Clashes with northern barbarians happen *all the time* in Chinese history. They were continual pests to every leader every time. While there have been times where this was the focus of Chinese attention, this was hardly one of them. The main story was the conquest of China. The northern barbarians were just the side-show. They hardly defined the era or Qin Shi Huang-di.
The guy was a monument builder by the way. There was never a chance to make a monument 10 times bigger he never took. There was never a chance to build a highway or canal he didn't want to take (usually on the way to conquer someone - I suspect his troops were as good road and canal builders as they were fighters). Ergo - his response to the perenial barbarian problem that's plagued every Chinese ruler since the beginning of Chinese civilization was hey's let's build a massive wall. However, the only "protection" he was obsessed about was to make sure he didn't get assassinated. The guy was a paranoid. "The not die and not grow old" potion. His main obsession though was unifying China and centralising control - this was his passion, this was his reason for living, this was what he devoted his life too, this is what every single TV drama and movie on his life concentrates on (they usually show the Great Wall and the Terracotta soldiers as an add-on near the end after spending most of the show on his childhood, conquering and organization). This was his legacy to history - unifying China under central control for the first time.