This one is finicky. But it can be good if the enemy is camping the upstairs of the Castle.

Go to the Dodgem entrance by the Merry-go-Round (Carousel).

Go down the long hall of arched window/doors, you will see a blue/black bumper car leaning against the hall wall (but in the bumper car area) at the third arch on your left (second open arch).

Get on the small white wall behind it, jump onto the back (blue part), squat and move onto the highest part of the black bumper, stand up. Face the mesh ceiling in front of you and walk towards it and jump as you get to it (it may fail to work).

Your guy will just pop upstairs, he doesn't climb.

You can now assault the upstairs of the castle.

Note that the wall between the castle and the Dodgem area has an inches-wide ledge along it. There is a similar ledge by the other parapet area of the Castle that faces the Funhaus.