After watching one of those medieval knight movies I thought why not build one with Lego Technic.
This one was a big success. Having a big counter weight by adding a load of AA batteries in the back and a Lego rock with a marble in it threw it a stunning 9 meters. After tweaking it a bit the rock even landed at the wall. So taking it outside I reached 12 meters. That was the max. This is really a fun play.