Maybe aside from siege weapons, there were other attacks as well, like magicians and else.

Only big cities like “rising sun city” have the barrier. If I remember well only 5 main cities

The Rising Sun City has two layers of barriers: The obsidian wall and the dark elements formation/array. Just the obsidian wall, if I remember correctly, can block human's siege for a century. While the dark elements arrays is an "ancient" array which the demons used against the alliance of 6 races. Each of the demon's main cities have the eye and main formation of these array while the small cities around it have peripheral arrays which depend on the main array. Once the arrays (in main cities and small cities) are repaired and have enough dark elements, it can be activated. But, I guess the five animal only planted the three grass in those cities without activating the arrays because SYX hasn't had time to teach them about it.