The Trebuchet was the one of the most devastating siege weapon ever created. It could hurl large objects with much more power and distance than a catapult. Now, you can build your own miniature trebuchet out of LEGO® blocks.

[edit] Steps

Build a base. This must be a 48x36 flat LEGO map. you may add the little minifigure who is to operate the trebuchet if you like.
Build the support struts. These should run parallel to the sides of the base, and meet at a point on the centre line of the base, at the edge. All of the force of the moving part is supported by your base, so build it very strong to move all parts!
Insert the fulcrum. (The support, or point of rest, on which a lever turns in moving a body.) Make sure the load arm DOES NOT swing freely back and forth, so design your fulcrum with this in mind.
Place the arm. The counterweight will go on the relatively long side, and the sling on the other.
Experiment with a counterweight weighing between 2 and 7 pounds.

[edit] Tips

Experiment a lot. See what tweaks you can make to give the payload the most distance.
The sling is the most important part of the trebuchet, as the rotational motion caused by the load arm is what gives the trebuchet its great power.
The counterweight must be much heavier than the object you are hurling.
For extra fun, use a LEGO minifig as the payload.