Content Rating Support

I would appreciate some help with setting a rating for the opening chapter of my fantasy novel ‘King of the House Elves’ bitmen number #2232303.
I rewrote this chapter to recently after being advised that my first draft might be best rated 18+ because of threat and violence as it describes a palace in a fantasy kingdom being seized by a monster. I had been hoping that a rating of 13+ might be suitable as I wrote the work aged 16 based on my enthusiasm for fantasy books and films enjoyed by young adults, including the Tolkien movies that are rated 12+. I don’t feel that the violence level in this chapter is anything beyond that in movies like the Hobbit or Lord of the Rings and I worry that a 18+ rating would prevent other teenagers who might enjoy my story from reading it. I’ve currently veered on the side of caution and put an18+ rating on the story but I would appreciate some advice on whether I could drop it down to 13+.