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.