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, and all within such a short timeframe (the traitor attack happened about a week after visiting the witch). I looked in the editor, the altar is on an island, there's a portal leading to it, but where's the entrance, that's the question. And why is this quest given as a secondary one, but failing it leads to a loss? Maybe the main character could have stayed in the castle, but they drag him into an open field, so it's a problem. In general, it's strange, although maybe I didn't do something right?