Paul posted some thoughts on these things at Benno's and on his blog, might help:

http://bennosfiguresforum.com/viewtopic.php?f=102&t=21021

https://paulsbods.blogspot.com.au/2017/10/orc-catapults.html

Odd thing, if you turn the catapults round one way it looks like the orc crew have nowhere to stand. If you turn it round the other way I think the Troll might be bashed on the head by the throwing arm. But I don't like trolls so That's a good thing isn't it?

Think I'll have a look at the film later on to see what they look like on there.

That's what I ended up doing. After a lot of trial and error, I finally figured out how to build them. It just takes a long time for all the gluing which must be done in stages. I will say this though, after they dried I set up my orc army and it looks ****ed impressive.