From the Introduction:
Trebuchets, masterpieces of engineering these weapons are among the most devastating weapons of siege warfare. This intimidating weapon has been constructed on top of a small artificial hill, raised for just this purpose. The artillery emplacement on top of the hill has been surrounded by circles of tapered poles and earth filled gabions to protect it from infantry and cavalry assaults.
The wooden frame of this catapult rises to a height of almost 50 feet, making it a distinctive landmark. Its counterweight is cast from iron in a single piece, making it exceptional among its kind. It is reloaded using a winch positioned at its back end. The winch is turned by two levers, each large enough for two or even three people, still only the planetary gears inside the winch allow the crew to raise the massive counterweight.
The trebuchet's construction has only recently been finished and the engineers are still present at the site as a few stones, each weighing more than a hundred pounds, have been unloaded to start ranging its shots.