Slings and Big Stones
There's a Trebuchet Club thread on the Nano forums, open for all writers who have a trebuchet in their novel, in whatever form (real, model, discussion topic ...). It reminded me that I have some pics of trebuchets I took at Caerphilly Castle.
Traction trebuchet in the foreground, a counterweight one in the backThey're replica, on display on the south dam platform, part of the outer ring of fortifications. The big brother of the Roman
ballista, trebuchets are basically giant slings that could cast rocks or later, iron balls, into the walls and roofs of a castle or town. Traction trebuchets work by human power, with several men pulling the casting bar down for one fellow to load the sling which was then released and would catapult the stone into a target - hopefully.

To the left: A fine example for the traction trebuchet is this minuature from the Maciejowsky Bible (c.1240). I found the picture on
this site, but it's pretty well distributed on the net. Carl Pyrdum from
Got Medieval has another one from the same book where the painter got a bit tongue in cheek about what can go wrong when firing a trebuchet.
Counterweight trebuchets had a box or basket filled with sand or small stones to make the pulling down of the throwing bar easier. One or two men could work the ropes running over a cogwheel, where it needed half a dozen at least with the traction ones. Thus, counterweight trebuchets could come in even larger versions.

On the right is an example for a counterweight trebuchet, a drawing from an illuminated letter from Edward II's charter in Carlisle, describing an event from 1316 when the Scots under Bruce laid
siege to Carlisle which was defended by de Harclay. It shows a number of interesting details, like a dead archer on the ground, which proves that the siege engines were pretty close to the castle. Also, the rope holding the counterweight was obviously cut for release - look what the guy with the hammer is doing.
The picture is from the same trebuchet site linked above. Thanks to Alianore for hunting down info about that letter.I'm not good at physics; I leave that to Constance, lol. But the trebuchets, with their casting bars that held the slings up in the air, in front of a castle, make for some fine pictures. The Caerphilly ones are sometimes fired by reenactment groups. My guide book has a photo of several guys hanging on the traction trebuchet to get it down for loading.
Another shot of the traction trebuchetAnd yes, I'm probably going to have a trebuchet or two in
Kings and Rebels. Though I'll leave the details to siege engineers, and none of my characters happens to be one.