When he realized that what he was looking at was the castle of Sariachal, Koutarou felt such shock it was like the world he was on was crumbling apart.
“... Now I get it... that's why they came to room 106...”
However, by accepting this, the pieces of the puzzle started falling into place. Using this truth to reconstruct the collapsed world, light was shed on parts that were previously unseen. The fact that so many girls had invaded Koutarou's room was not a coincidence.
According to Dark Crimson, the Folsarians were once People of the Earth that separated from the other tribes. And now that they had found the castle of Sariachal, they understood that Folsarians had actually come from the past Forthorthe. In other words, Folsarians and People of the Earth were actually descendants of Forthorthians.
In Koutarou's final battle in the past of Forthorthe, Maxfern and his group of alchemists, and Grevanas and his group of magicians, had been blown away by the super-space-time repulsion shell. And the two groups had drifted through space-time and ended up in the past Earth and Folsaria.
Maxfern and his group that arrived on Earth created their own culture based alchemy. The ruins at Koutarou's part-time job were most likely that. However, for some reason they had vanished. The virus that had been blown away with them might have been responsible for that. In that case, only those with blood mixed with Earth's humans would have survived. Reason aside, Maxfern and his group had indeed vanished. And those that survived inherited the technology and culture and developed on it, eventually becoming the People of the Earth.
Meanwhile, Grevanas and his group of magicians had reached Folsaria. They regretted turning their fangs on the royal family alongside Maxfern, and created a kingdom without a king. That was the magical kingdom of Folsaria. Combining with Forthorthe and Sariachal they got Folsaria. The capital being named Thorthe was made by just removing ”For” from Forthorthe. Everything made sense.
The People of the Earth began declining, and there was internal conflict within Folsaria, and eventually both of them reached out towards Earth in order to invade. In that process, Sanae was turned into a ghost, and Yurika became a magical girl. Theia and Ruth arrived at Earth by adhering to their customs, and Kiriha appeared as the People of the Earth's vanguard. While they all appeared to be completely unrelated at the start, they had a common connection in the past Forthorthe.
“... In other words, this is all because of me. This was my battle from the very beginning...”
With this, Koutarou was given a new viewpoint. The girls appearing in room 106 all originated from Koutarou's actions. In other words, this entire situation was all Koutarou's responsibility. Up until now, Koutarou had acted to help the girls, but it was different from now on. Koutarou had to take responsibility for what his own actions had caused.