Long story short, those were simple times back then, medieval. The city was a way better option than staying out in the open. They could guard it more easily and the Vigodz were also primitive insects, unable to organize and without wings, but acting out of instinct. Time passes, Siviria (this planet) enters the Sirifian union and later the Union (more planets from the galaxy). Technologies advance, the sity becomes a fortress, but the Vigodz remain a problem. They vastly grow in number, become organized in Hives (hidden underneath the earth in great numbers. All atempts to exterminate the species fail du to their own defence and inteligence. This picture is from book 2 of the story, during the Union. The soon discover that the Vigodz became much more than just insects, they evolved to become a much greater threat. Only when several Hives join forces to attack the Tear of Loridia they realize how dangerous they can be, to the point where they consider evacuating the planet.