"Were those… giants? Onis?" Billy thought when the memory ended. "I saw some mountains in my memory, and they seemed familiar like the ones around Jeannette's former base… they were like the mountains of Neles state…"
Billy confirmed once and for all the enemies will come from other planets. Nevertheless, he never imagined that he would see such a thing as a flying castle in his life. Stupid amounts of mana would be necessary to pull that off.
Regardless, considering that Zenis had that cannon, perhaps he had seen something similar and prepared for it. An automatic anti-aircraft defense system… maybe Billy had to make a few of those because there is nothing better for flying enemies than structures that can't move and targets on the ground.