Well, catapults are the earliest siege weapon, so they should be the weakest no? anyways, most of the time if your using pults, the city your targeting has no walls. If it does have walls, pults are still good, provided you have some swordsmen to attract/absorb the city defences. Even then, you might only get one shot. You better have three of them if your laying siege to Babylon, or just wait for better tech and upgrade. if no walls, the archers are better because they can move around easier, and are better overall in the early-mid game. Sometimes trebuchet are a great unit, sometimes not. You need to protect them from harm. cannons are when things get interesting.