trebuchet
A trebuchet was a siege engine used during the middle ages (up till the 16th century) to attack fortified locations. It was used to hurl large stones either at the masonry defenses or over the walls to attack the citizens of a town.
This model used the following materials:
Wooden Plank Mold #220
1 paper clip for the sling attachment, the counterweight pivot and the arm pivot.
8mm thread for rope
Some old .380 bullets used for reloading
Some cereal box cardboard to help strengthen the throwing arm.
Size: the trebuchet itself is 6" X 8" X 12 3/8" tall. The base it's on is 12" Square. 12" X 12" (the base) X 12 3/8" = 36 3/8" total.
This project came about because the ruined aqueduct project I originally planned on doing didn’t work out too well (the word “disaster” comes to mind) so it had to be abandoned. I looked around for something that could be completed on time (this was Wednesday Aug. 13th) and realized I still had these plans from last year (it was going to be another entry in the restricted mechanical category but I never had time to do it due to the category being announced so late).
Day 1: assemble all the sub components. The arm, frame components, and the bottom part of the frame
Day 2: assemble the frame, create the counterweight basket. I had to rebuild the arm at this point because it broke when I was test fitting the pivot. I rebuilt it with a strip of cereal box in between the plaster “boards” to help give the arm more strength after I drilled the pivot hole.
Day 3 (Last day for entries): Attach the Arm and counterweight (at lunch). Paint the piece and take pictures (in a great rush with the paint still wet).
This piece is fully functional and will throw blocks. The best I've done so far is only a foot or so for distance but I was getting better. I have finally run out of time for testing since it's less than an hour to the deadline as I write this.
You can see some attempts to use the trebuchet from the following links:
First attempt was without any weight in the counterweight bucket and wasn't very successful. After that attempt I added 5 .380 bullets for weight and it worked better.
Videos are in AVI format
First Attempt
More Weight
Best one so far
As the videos show this is functional but I have yet to figure out exactly how it works the best. It's possible the sling needs to be changed or maybe add more weight.
The sling is made from an old sheet I had laying around to be used as rags. I just cut a slice off it and tied it to the thread and hooked the thread through the piece made from a piece of paper clip.
To help strengthen the arm I cut a piece of cereal box the same size as the large planks and glued planks on each side of it. I then trimmed the cardboard down with a hobby knife. The holes in the arm and counterweight basket were drilled using an old drill bit.
The painting was done using Americana acrylic paints. The base coat was Milk Chocolate brown and then a dry brushing of Honey Brown.
All pieces were cast from Hydrostone. Merlin's Magic or Excalibur would probably be better for this kind of delicate piece but Hydrostone has worked out fine.
trebuchet ready for firing
An overall shot of the trebuchet with the arm held back for launching.
A close up shot of the counterweight box and the pivot point of the arm.
A shot of the base and the sling in the held back position
A rear view of the trebuchet in the pulled back position
The arm has full range of motion. Here is a shot in the halfway position.
A shot of the arm in the fully upright position. This is where the trebuchet ends up after you fire it.
A close up of the pivot point for the arm. The pivot is nothing more than a piece of a paperclip I straightened and then pushed through a hole in the arm I drilled using an old drill bit.
A close up of the sling. Nothing special here. Just some 8mm brown thread tied to the ends of an old piece of sheet. The sling will probably have to be redesigned since it isn't letting go of the projectile properly. I'm thinking I need some kind of quick release for one side of the sling. 
A shot of the counterweight basket. Inside you can see some of the bullets used for weight. Originally I wanted to put a few blocks from the Dungeon Builder mold to cover the weight but I'm still experimenting with the weight to see what works best.