Sounds like something ideal for DM hand-wavium, personally. What're you trying to do with one?
'Hand wavium' doesn't work very well when you are using them to shoot PCs.
Or for that matter...
If you're trying to take down a dragon, DM hand-wavium for the DC to aim it, standard action to fire it, standard action to load it, and DM hand-wavium for how many bolts/rocks/cows have to hit to kill it.
... if you are fighting a dragon. Players will never again take a dragon fight seriously if you hand wave its death.
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It's not clear what Piratecat wants to do with these 'siege engine' but most of what has been named can be used for anti-personnel purposes so it can be an interesting thing to add to a battlefield.
Catapult; They can be used to lob all manners of antipersonnel stuff, especially in D&D where you can add alchemical compound to the mix. But you can't realistically target a PC with a catapult. You just throw it in a mass of enemies and hope something good happen.
In my game, I used off map catapults to randomly blast a part of the battlefield each round. Even enemies could be caught in these shrapnel blast, alchemical fire etc. (Which is a typical trope to highlight the ruthless nature of the overlords who don't care about the fodder). Added quite a bit of chaos to the battle. I numberered several 3x3 zones and would randomly blast one of these area. You get to scream 'incoming!' when you roll which is fun.
'Big Honking crossbows' are called Arbalests. They could generate as much as 1200 pound of pressure and could only be spanned through mechanical assistance. These were basically the sniper rifles of the middle age. In direct fire, they had much more range than a longbow and a hell of a lot more punch, which is saying something.
Something like, superior weapon, +3, 40/80, 2d12, high crit would be about right but then you hit the weapon in the gonads by requiring a standard action to load, thus halving the fire rate compared to a normal ranged weapon.
A special rule should prevent it from being kept loaded at all time just in case of an encounter.
Not very useful as a PC weapon, but can result in interesting fights where a damned sniper type opponent is shooting at you from 70 square away while you slug it out with the main force. Wee.
The Roman Ballistae
This is something that certainly could be wielded fairly directly against the PCs to throw all manners of projectiles. For example, just use alchemical fire from AV but boost the range, make it Area 1 instead but use a quick dirty rule to see if it landed in the intended square or if it missed by a few squares (Like; 1d6-2 square in direction 1d8)
You can also use it as a really 'Big honking crossbow' and just fire the traditional bolt; apparently there are accounts of specific human targets having been picked out by this kind of weapon. I'm not an historian, I haven't read the historical account, but assuming it's true, I find that highly impressive, I gotta say! Sniping out a cavalry commander just before a charge with that thing screams; 'Give that soldier a medal RIGHT NOW!' to me.