The human alliance, who originally thought that their plan had succeeded, found to their dismay a sad truth after the dust had settled – although they had killed Eviltusk, the biggest threat to the city walls, they now had only two or three hundred people left. It was far from enough to defend the sixth wall!
The city wall was an inanimate object that had could not attack back by itself, so even though it was tall and imposing, even a three-year-old child could conquer it, given enough time. Therefore, a wall was only useful if there were enough soldiers defending it, just like tanks and fighter jets would only be useful if there were people controlling them.
