The forest ruins are located in the Great Woods almost directly northwest of the Castle.
Talking to (or stealing from (or attacking)) the Evil Cleric in top-left (behind the secret door) is required for the first mission. They will remain there even after completion.
The Evil Clerics in the north-center are not hostile until the you interact with the mission-related cleric, after which they'll beeline for your position.
The walls are climbable, but only in spots where it's 1 tile thick (e.g. to the north-east, select tiles along the west wall, etc).