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While enjoying some tea with desserts, I was able to learn more about Erika's mother. It seems she had participated in the wars against the Demon Armies, and had even gone into a Demon Tower herself, the nearest Demon Tower from here, the Fifth Demon Tower which is located within the Ever Lake City. Visiting Demon Towers was more dangerous than Dungeons and involved whole squadrons of people of over twenty and more.
Demon Towers are Sealed currently, but as their Seals weaken, their floors open up, filled with demons coming straight from the Realm of Hell. Each Tower represents a certain sin, involving an Archdemon. There are Seven Towers for all seven sins. The nearest Tower is the Tower of Gluttony, whose ruler is Beelzebub.