Hm...those questions are tougher than they seem. Right away one of my favorite things was the ghost cast, and how they were more than just demented tormented souls. One of the other things (Which I'm fairly certain is a staple of most horror stories) was that the Chateau clearly had a backstory and it was revealed bit by bit while we went through it.
As for the scariest parts, having Darius murdered right in front of us was a great way to make the horror clear while still leaving us in the dark for a bit. It's kind of hard for me to think of what else about it was scary mainly because I don't get scared during stories I'm reading for the most part. But if you wanna bounce any ideas off of me let me know, I'm happy to help.