In the annals of human history, what is negative and positive about building walls between peoples?

In ancient China, Great Walls is necessary to block northern minority groups away for China dynasties. If the regulators didn't build Great Walls, the nothern nomads could invade China at any time they want from the long remote north border.
However, this kind of passive defence could not 100% get rid of organized large-scale attack. And the construction progress that lasted for thousands of years greatly wasted money and manpower.