This is a late Roman Onager, recreated in Lego, as described by the author Ammianus. The "Onager" or "Scorpio" was a late Roman stone-throwing machine which used a torsion spring to propel a projectile several hundred meters with very high precision and accuracy. The arm was stopped with a mattress or mound of dirt/sand, rather than the commonly depicted crossbar design. The reason for this is that a crossbar prevents the full power of the spring from being fully utilized, and would also break the arm.
This set is made of 97 pieces and took me about an hour to design and build. The real set would have a rubber band and string with which the arm can be drawn back and fired, but this can't be recreated in the digital designer (as far as I know). It comes with 2 minifigures.
The two minifigures, called Tragularii (artillerymen). They wear a padded underarmor called a Thorocomachus, which was made of thick linen. This was never used as combat armor, but would be ideal for their role, being light and positioned out of range of most missile weapons.
This is currently a draft in Lego Ideas and I hope to submit it after my other two sets have been allowed.