Abdication

I played for about a month and a half and failed the traitor mission. I still don't understand how to get to the altar, especially within such a short timeframe (the traitor attack happened about a week after visiting the witch). I looked in the editor, and the altar is on an island, with a portal leading to it, but where is the entrance? And why is this mission given as a secondary one, but failing it leads to a loss? Maybe the main character could have stayed in the castle, but he's dragged out into the open field, so it's a problem. In general, it's a strange situation, although maybe I didn't do something right?