So it was mentioned several times that if a beast from the surface tastes of smells  specifically human blood they go into a frenzy and try to kill as many people as they can. This got me wondering how on earth could behavior like that be a naturally occurring thing. My own personal theory is that the surface beasts where made to kill humanity and someone or something, the "builder" mentioned more then a few times, made the spires to shield humanity from these creatures.