Northern Fortress was a frontline of Star City in the north. It was a fortress to defend and keep a watch on the zombies' movement.
The north of Star City, there was a huge amount of zombies gathered. Before the catastrophe, that place was a flourishing country called Russia. However, after the catastrophe, Russia left with ruin as the people there moved out to Moon City.
Later, Russia became a place where the zombies gathered. There was a zombie that gained intelligence. Not only this zombie gained intelligence, but it was also very strong.
This zombie was in the level of Emperor Class Hero and Legend Class Ranker. An Emperor Class Hero and many Gold Class Heroes fell under this zombie.
Because of this, the Hero League and Central Government built a fortress in the north to monitor the zombies.
I read a comment saying that why did Heero use that technique to kill flies? That's because he got emotional as someone like Feng Fennu tried to steal his wife with their authority. If you read the prologue volume, he lost his childhood sweetheart under the attack of Specter and lost his lover under the attack of Evil Dragon. So the mc is getting emotional when there is someone trying to steal his wife.
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I will pick a question or inquiry randomly in the comment section. If you have something that you want to ask or there's something confusing, you can ask me. I will choose one and answer it in the next author's thought ^.^ No spoiler ^.^
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I will post the next chapter in five hours.