This is probably the most informative thread I've found on the subject (but it's old): http://www.twcenter.net/forums/showthread.php?t=39031
I noticed in my single player campaign that the siege battles were somewhat Roman feeling, but I only hit a couple small towns with wooden walls. A battering ram opened the door pretty easily. Since these were small towns I fired up a custom battle and threw in the largest city I could with a stone wall.
I pitted my 1800 men, I think I had six missile teams; four yumi of varying kinds, one archer monk and one arquebusier unit, then one pike unit, one no-dachi unit and a couple horse units, general included.
Defending against a force of 2400 men. I thought I put together what was a reasonable mix of what should have been a decent assault team. A lot of upgraded Samurai, no less than four yumi teams, a slew of just random melee units that I thought would be good at assaulting the walls. I gave them one ram and two ladders. The results were surprising;
* Neither ladder unit got anywhere close to the walls. Maybe within 50 meters. My yumi and especially the arquebusiers just decimated their forces. Clearly the ram was more effective. And really I'd expected to burn down their ram and stop their ladder units cold.
* The ram made it to the gate with flying colors. I was hitting it with all sorts of flaming arrows but there was NO damage whatsoever to the ram. Frankly I was shocked. Am I missing something here? After my abject failure at burning the ram before it got to the gate I tried to rush my no-dachi unit out to kill off the stragglers to stop their busting down my door. They wouldn't go out. I guess that's not a big deal. The ram broke through my gate and my pike unit and nodachi unit and general easily repelled the small assault force they were able to bring. [EDIT: Nevermind, I see that I actually have to enable flame arrow. After I did that the ram didn't stand a chance...]
* I was half expecting a mountain fortress when I started this little test. Obviously it's flat, and I assume that is simply a limitation of the RTW engine? Or would it be possible to build custom maps with "tiers" and multiple walls? Likewise, I was expecting an inner castle courtyard to be the capture point to hold for a victory. Is that also just a limitation of the engine?
The end result of the battle. Yes, the ram breached my gate, ladder teams were worthless. I believe they lost nearly 2300 men, 700+ to the arquebusiers (that's scary!). They never came anywhere close to penetrating past the walls. Obviously it makes far more sense to starve out the defenders, but I was just curious what the limits are to this engine.