From more than 100 years ago, human natural enemies appear, have overwhelming power gap between them and human human face of extinction, the surviving humans built from maria, Rosetta, sheena three walls spent 100 years in the life of peace. In 845, however, with the super giant and armoured giants of suddenly appeared, with daily life and the wall destroyed, human abandoned, the wall of maria, lost 2 into population and one-third of the land, areas of human activity also retreated to within the walls of Rosetta. In 850, the wall destroyed by mega giants appeared again, the giant's offensive began again.The giant to creature had no interest in anything other than human, it is only the action principle of eat people. But considering the giant in the absence of human environment also survived for one hundred years, presumably they absorb food, and it is not necessary but for killing. However to push human to now this situation is the biggest cause of giant that astonishing vitality, humans have to master the art of artillery, but with only this is unable to compete with the giant, even if the head was hit fly, giant still not die. In addition, although there's a difference between individuals, but as long as 1 to 2 minutes, lost head will return to original state. Only a down with a giant method, only after the neck suffered serious damage, the giant will not be able to regenerate, finally died.望采纳