Making Siege Weapons Interesting

[Shorticus]

So, I'm gonna be setting up a low magic (sort of) campaign based in a pre-gunpowder medieval fantasy continent, max level of 8 for PCs, somewhat higher for very powerful NPCs. I want to make siege weaponry sometimes appear in the story, such as during sieges and the like, and I'd love to be ready to unleash some kickassery when my players inevitably ask "Hey, can I use the ballista?" Or, y'know, when I want to fire a trebuchet at the wall the PCs are standing beside.

What are some ways you'd suggest to make them more useful and interesting? I know the Pathfinder siege engines are generally considered to be boring/lackluster. Any ideas?

[avr]

You might want to make them more effective against objects/structures, possibly unrealistically so. Making them do more HP damage just puts adventurers out of a job.

[Nodrog]

I disagree that they would put adventures out of a job. As balista, catapults and especially trebuchets are terribly difficult to move. They require a crew of probably 4 or more to function reliably. Several draft horses or oxen to slowly drag them from battle to battle.

They would make good plot hooks, as the king might ask the players to try and stop or sabotage the weapons before they are within range of striking the castle or town.

Or perhaps the general has heard of this "terrifying" new weapon and want a stealthy party to go steal plans for it, or just steal the whole damn thing.