Currently, Siege Engines are probably more accurately "Anti-Siege Engines". Static. Unless you're embarking right next to a settlement that you wish to take on, militarily, it is only when besieged (or, to be honest, when anything from a marmocet to luckless kobold a comes trolling across your Field Of Fire) when the Engines come into their own.
Maybe when you can wander off-map with a small army (in nominally Fortress mode) to stem the tide of goblins at source, a catapult composing of the three catapult parts, a couple of axles and maybe a mechanism for good measure might, either carried off in bits or towed by a tame elephant/couple of stout oxen/a whole string of ponies could be just what the doctor ordered...
I don't know if the only real reason why siege engines can only be 'aimed' along the orthagonals is down to the awkwardness of depicting them facing at any other angle (even 45°), but I see no reason why a siege engine could not have a more realistic field of fire. And perhaps a reactive one, even a possibility of striking an enemy through actual aiming, rather than luck (and if it isn't just luck, then I'm using them wrongly!)
And I think trebuchets (would need much the same materials as a windmill, currently four logs, plus a mechanism or two for priming/releasing) would add to the menagerie of large-scale weaponry. One of the aforementioned size would probably be a "build in place", even if you could wheel the others around, and would have a minimum and maximum range, within which a (suitable skilled) operator could reliably drop whatever was launched down onto. Perhaps, especially if rocks, it could be made to have a cave-in effect upon the target zone, at least at the last Z-level. Visually, there would be a 'shadow' rock travelling along the ground Z-level (up and down the contours of the land) until it reached its aimed-at (or misaimed-at) point, at which point the cave-in-code could be appropriated to give cave-in-style injuries on anyone who happened to be on that square and choking-dust effects to the nearby surroundings, with no 'ammo saving' features, like the current catapult can be exploited for. (Could not be used in a tunnel, or would require a nod to a parabolic-style 'headroom' check along its path, at least. But
could be used over hills.)
In some ways, I'm ascribing some ideas to the trebuchet that
should really be relevent to the catapult as well, but we're used to how those work, now, so it might be a revamp too much for players to accept.
The idea that there are different scales is interesting. I'd say
- Ones of 1x1 size (a single "<foo> component") that can be manhandled around the field of battle by a kind of 'mortar team' or HMG squad, if that's not anachronistic for the attempted-for era,
- 3x3s that can, though needn't be, mounted on axles and dragged and must be assigned to be rigged/de-rigged, alongside some necessary "emplacement" designation. Perhaps (b)uild, s(i)ege engine, (C)atapult position, and selecting as the 'building materials' a currently mobile catapult unit. After hauling to the position (might be best to wait until Beasts Of Burden can be utilised by non-traders) the 'building' is constructed and usable as per a standard built-in-situ engine, but 'deconstructed' merely makes it mobile again (to drag to new location or allow to be brought back to a suitable designated zone/meta-stockpile when unallocated and no hostilities occur?)
- 5x5s that are purely static [de/of]fensive Engines Of War. Trebuchet version is capable of raining multiple (3-5?) boulders down in a spread pattern (on top of the aiming), a double/triple spread of standard ballista bolts for area protection coming from the mega-ballista (c.f. 1x1 ballistas-equivalents which might fire a spread of 5-7 crossbow-type bolts into a group of hostiles in one shot, but at the expense of the 'crew' protecting themselves) and something equally relevant for the catapult version. But at the expense of time to build and immobility (possibly even no rotational component, aside from a 45°-each way arc of fire, after having designated the directon it should face).
Note that I'd expect "Vile Forces Of Darkness!" to be capable of at least
capturing your equipment (the operators would run away) and maybe even bring in 3x3 mobile weapons of their own. Mobile siege towers might be their preserve only (after all, if the dwarves were taking the fight to others, they wouldn't be 'below' using sappers' tunnels instead of siege towers) but, for the sake or argument, would consist of the same components that make a 3x3 mobile (or just the components of a wagon) plus a number of units of material (mainly wood, some metal?) for the walls and the stairs and a 1x1 ramp (i.e. bridge). The heavier the components, the slower it should travel/more troops/operators/slaves/animals it needs to get it into position. Maybe it could be constructed (off-map?) in response to a particular need, if the software agents behind the hostiles can be asked to work out what height wall/cliff they'd like to be able to climb over, and thus larger and slower moving) towers could arrive in response to your having a 4-high defensive wall with fortifications that you were hoping would rain bolts down on everyone from. If the ramps(/bridges) of the tower were capable of copying their standard built cousins and atom-smashing at least Fortifications, once brought within range, then it would add a dimension to defence not heresofar necessary to consider.
But the above is a massive amount of change, a stream of consciousness more than a focussed idea, just there to dip into. (As long as when you dip in you grab the balancing aspects, so the overpowered items are more difficult to make/keep/use as well as more dominating when they are in use...)