Construction techniques aside, what was siege technology like in the Roman era? Did they build walls to resist heavy thrown rocks, or just arrows and infantry?
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.
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.
Greek and Roman catapults were lighter than medieval trebuchets, using tension or torsion springs rather than heavy weights, but were reasonably capable of hurling rocks well enough to take down a wall eventually. I don't have an example offhand, but the Romans were more than happy to build thick walls to resist sieges if they needed to. My impression is that, fortification-wise, Romans preferred walls and ditches rather than single highly-fortified buildings like castles.
On the other hand, Roman architecture was such that they did not need very thick stone walls to support high buildings.
