Those of you who look closely at the LBBs have probably noticed that they make occasional comments about catapults, mostly comparing things like giants tossing boulders to them. However, the rules for actually using catapults are nowhere to be found. You were expected to use Chainmail.
I'm mulling over some simple rules for catapults. I'm lumping all siege missile weapons together under the name "catapult", because I've learned that, contrary to what I believed for 40+ years, that's really all "catapult" means; what I've always thought of as a catapult is specifically a mangonel or onager. The original catapults were basically giant crossbows.
First, catapults are going to use the same attack roll as ordinary combat. It will be treated as an attack by a 0-level man, even for Fighters, unless you toss in a Siege Fighter variant who fights as a 0-level man in melee, but gets a level-based bonus for siege engines. An experienced siege crew can get a +1 bonus to attack, as can characters experienced with siege combat.
Second, these are not one-man weapons. My simple solution is to equate crew size and dice of damage: a 2d6 catapult requires a two-man crew to operate, while a 3d6 catapult requires a three-man crew, and so on. Plus, it takes 2 turn per die of damage to set, load, and fire, with each extra crew members (up to double crew size) reducing this by 1 turn. That's probably not accurate, but who cares? It's reasonable enough, simple, and distinguishes catapults from melee weapons or bows.
Catapults that fling stuff as opposed to firing a single piercing missile also have an area of effect equal to 10 feet per die. Rather than laying down a circle of some kind (as Chainmail suggests,) I'd just have everyone within range of the target roll a d6; if it matches one of the dice rolled for damage, they are also struck.
I'd simplify range to 15" per die of damage. Minimum range for mangonels, onagers, and trebuchets would be half maximum range; you can't fire them at close targets, but you can fire them over the heads of people in the way. Catapults that fire javelins or arrows can be built so that they have no minimum range, but they won't be effective against fortifications, only against people.
Rather than work out exact specifications for various siege engines, I figure most of the differences affect non-combat issues. Tension-style catapults, where a springy firing arm is bent to create the force behind the projectile, would be easy to build, but harder to repair; counterweight-based systems are the opposite.
I'm not necessarily finished with catapults, but fragments of this post have been sitting in my drafts for two months now, so it's time to release it into the wild.