If you go back a bit in this thread, you can see my points in messages 31 and 42 why catapults would not be very effective against the sort of barricades that would be, in essence, just piles of destroyed buildings/vehicles pushed into a defensive ring around the city center. This sort of rubble abatis would not stand up against explosives, but would do pretty well against any non-engine powered ram or explosives.
Plus, as I mentioned not to long ago, another problem is that with catapults, you can't always expect to hit your target; you don't want to overshoot into the city since you might end up destroying some very valuable stuff your trying to steal in the first place.
Plus, since for much of the early game (say the first 20-40 years) you're really not going to have access to many tracked vehicles with the power to plow through such a wall so for the most part, the barricade is probably mostly the crude abatis with a trench/ditch outer ring along with wire obstacles. This is enough to help deter infantry and trucks (concertina wire does wonders on wheeled vehicles as I can attest by personal observation) but in the face of something like a heavy duty Caterpillar dozer, would probably fail pretty quickly.
Hence my whole point of steam engines. They may not be as effective as gas/oil vehicles, but if you make some early-ish in the game, you could use the to great effect when attacking cities.
Perhaps there could be a "National" unit that is built like a FfH hero unit where you get your steam "Killdozer" available somewhat early. It's tough, slow and really only works against cities. Is it worth the time and resources to build it instead of 2-3 buildings or lots of cheaper units? Depending on your Vision and your Civ, it may be if you are a conquering type.