In the image above, we can see an illustration of the use of the following two objects.
Definitions from The Scriptorium (http://thescriptorium.co.uk/glossary.php)
trebuchet - siege engine operated by a counterweight
belfry - a mobile wooden tower used to transfer troops onto a wall
The description provided on the source site for the image above describes the machine hurling rocks on the attackers as a catapult, but it looks more like a trebuchet to me. From what I understand (though I am no expert), catapults work on a concept similar to a bow or a spoon when hurling peas across the room (bending and then releasing). The trebuchet uses a swinging counterweight to hurl objects. From what I have read, the trebuchets could throw farther and longer than catapults, which would make them my weapon of choice when attacking a thick castle wall.
Here are two examples for you to compare:
Which do you think is being used
in the picture at the top of the post?