Why?
Zemeth returned to the castle and saw that the second opening was about to end. It doesn't matter, he already got Ivy. Beginning today, the castle can no longer release monsters because the three sisters were already out of the castle. The golden dagger will no longer work since there is no compatible blood to activate it.
It's a good thing that he was able to save Ivy this time. He no longer feels sad and guilty. He willed his mind back to the present and scan the whole area. He was currently hovering in the sky watching his companions slaying monsters everywhere.
The biggest advantage of Zemeth and his team was their ability to hover in the air and attack the monsters from above. The monsters have no weapon to protect themselves. They only have sharp claws and sharp teeth to attack humans, aside from that, they can also run fast.