I have a few theories. Like everyone else, I believe the "love cupid" advised the girls to stay away from him. Also, I believe the girls are actually staying at the castle. They noticed when they entered that it was unusually clean for an abadoned castle, but Caius didn't give off any "clean freak" vibes. And at the end of this chapter Youki is shown wondering about the cleanliness. If a number of people were staying there, that might explain it.
However, that doesn't quite explain why he wants the guy dead, and why he needed the help of a hero. And why claim the girls are at a monastery. Something like "he's brainwashing them and holding them hostage" might work better. So maybe he already tried and his subordinates were defeated?