Well, on Regent, sometimes I don't bother with catapults unless i've got a non-barracked but reasonably non-corrupt city that isn't otherwise useful as a worker pump. After getting engineering though, I'll start making some trebuchets (not too many, actually, i should be building more than I do). They (catapults) can be upgraded reasonably cheaply to Trebuchets even without Leo's Workshop. They haven't been all that effective for me, but then I usually don't have too many of them either, and that's probably what makes the difference. Artillery is defintely at its best in a large stack.
Trebuchets are pretty useful, actually. On Regent, at least for me, you'll notice your artillery really turn the corner once you upgrade them to Cannons. It seems to me, at least, that the accuracy improves. It could also just be that I've got a lot more of those units by that point. (The Treb to Cannon upgrade is pretty cheap as well).
Basically, with artillery, at any technological level, quantity is its own quality. Since it's a random "hit and injure or miss" type of effect, the more you've got, the better off you'll be. Just don't forget to protect them with the relevant defensive unit (spears, pikes, muskets). At Regent I've usually been able to get away with not relying on it so much and not using/making very many until I get to Cannons, when my military is just so big that I just gotta use it on somebody (plus I have several cities that can crank them out quickly and in fact have bugger all else to build). That's when I step up my production in this department. Once I get magnetism I'll try and build some Frigates too, to supplement the stack some additional bombardment from the sea on a city I'm aiming to take when applicable (which isn't as often as I'd like. Oh well. One more reason to crank out the land based artillery

). But really, as soon as you've got a city that can crank them out at a reasonable rate, crank out catapults. Even if you don't need 'em/use 'em yet, think of 'em as tomorrow's cheap Trebs./Cannons.
Redlining the enemy beforehand (especially when trying to take a city) will save your horsemen/swordsmen's lives, and they will thank you for it, sometimes even in the form of living to make elite rank, and then pop a few military leaders/armies. And once you've got THEM, of course, it's a jailbreak for your military
