DireSickFish

I'm trying to take over the castle while murdering as few folks as possible. I've already come up via the sewers. Then disguised myself as a monk and got the key from the priest because I had freed the guy in the sewers. Now I don't know where to go because everywhere is full of guards! He told me to avoid the main hall and I feel like that is down on the first floor and the only way to go. Where am I supposed to go to sneak up on the leader?