Quote Originally Posted by Gemina View Post
You can't lose rank, but lose more the higher rank you are. I'm thinking it's probably best to do Castrum asap after hitting 10.
You don't lose rank but your mettle can go all the way to zero. Anyway, IMO you don't lose that much mettle that a few CE won't fix, something that I find myself doing from time to time to keep my mettle at max.

I actually did rank all the way to 15 before properly farm Castrum and honestly, at this point I think was not a smart move because I skipped successful Castrum runs.