Northen entrance(s) lead to the north tower, which houses the Heart of Sorrow. Southmost entrance leads down into the turret post (with a secret entrance into the castle proper). There are several entrances from the battlements/walls a couple floors up. It's a ways up but technically you can breach that. There is a big stained glass window in the audience hall on the second floor. Ground floor map, upper-right, you should see it. The main visible entrance is the front gates.
