[mangamuscle]

Originally Posted by Myssa Rei

The answer to that was actually mentioned in passing (as is the problem with anything concerning the Demon World) concerning the Magician's travels: the Outer Library.

Spoiler for Minor Details:

If you're familiar with the Librarian of Discworld, and his charge, it's basically that -- an endless library filled with books from ALL worlds, and ALL times/eras.

Incidentally, the place is also the home of the Maou and the Head Maid's tribe.

*read spoiler* Now I want to see an alternate version, where an evil Maou enchants Yuusha to be her pet and instead of new agricultural wonders, she brings into this era the horsing mount (a terrible weapon when employed by the cavalry), the gunpowder cannon (no brick and mortar fortification can resist) and the submarine (no naval vessel is safe).