how does the trebuchet and all the other catapult systems work? i don't get it can anyone help me please

Does thou not own the Main Rulebook? If so, you will find it in the first half of the book.

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"I am the Flying Spagetti Monster. Thou shall have no other monsters before me"
-FSM.

What part are you stuck on?

Hmmm...
GW might get upset about copyright if we post rules on here
when i cant find a certain rule i ussually go to an official GW store and have a look inside one of their 'open' copies of the ORB

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For God so loved the world that he gave his only Son, that whoever believes in him shall not perish but have eternal life.