@dskilly
He states, and I quote, "If I interrupt the
entire castle will be
blown to bits" and "Even if he's successful, a
part of the castle will crumble for sure." And that is what happened. The north tower collapsed. They absolutely did not gloss over that point because we see the aftermath, and we also see the King's room indicating that the main castle was protected. Plus, the one who shouted about there being people in the castle was the girl. Please don't consider your poor reading comprehension the same thing as a plot hole.
Did you read past chapter 20 or so? because he realizes that the reason the magic system is similar is because it is
fundamentally the same as earth's. The only difference is that this world uses a intermediary like elements, which lowers efficiency but adds a 'holy' attribute to all attacks(because the elements are connected to the Goddess), which is why the MC's attacks aren't completely op, since it results in a advanced fire spell being weaker than a normal fireball against Demons (with a 'dark' attribute)
This is chapter 7. Of course not everything is going to be explained in exposition right away- many of the problems you just expressed are purposefully left out to enhance mystery and set up for future reveals.
While I do understand having questions, your complaints seem like someone getting annoyed at not knowing what's going on in the prologue of a murder mystery book.