When you are on the Castle map go to the biggest castle and there will be a ladder at the back of the stair case.

Once you get up the ladder turn to the left and there will be a big hole.

Jump up on the wall and ease off, then go forward and there will be a red pole like figure.

Jump on it, then turn left and go straight, another pole connects to it.

It will lead to a green roof. Walk down that pole and jump on the roof.