[Retric]

Catapults and other torsion where during the Roman empire, though the thickness of walls also related to how tall they needed to be to be effective. Anything under 30 feet is easy to get over using simple ladders.

The Trebuchet is plenty old enough, but we don't have evidence of use by Rome.

[mcguire]

As far as I know, it depends on what you mean by trebuchet. Counterpoise trebuchets (using heavy weights) are definitely medieval.

[Retric]

Yes, the giant weapons most people think of when they hear the term trebuchet is medieval improvement allowing people to further scale up the design. A mangonel (traction trebuchet) is the older design, but trebuchet is referring any scaled up staff sling.