Go to the drawbridge of the castle. Go inside through there, stay left, make a left up the curved stairs on your left.

At the top of the stairs, turn around. You will be facing the roller coaster area. Stay on the lip of the top step and jump onto the 'wooden' wall on your right, jump ahead to the next section in front of you and then run and jump straight ahead to the top of the crenellated wall (outer wall of castle).

From here, either watch over the roller coaster area, fall onto the roofs of the 'port-a-johns' and watch the roller coaster area, or run right to the corner tower and jump onto the back wall of the castle (also crenellated-above popcorn machines). No-one will expect you up there as they pass through the castle.

Also note that from the upstairs of the castle, go to the 'dodgem' area and jump onto the remains of the 'roof' of the bumper car area. This should be obvious, but it is a spot you will probably get sniped at, so it is not great.