I did the test using the Britannia set of the Kingdoms expansion, and the culture was Northern European. I forget which map off the top of my head, and I only did one test. I'll try to do a bunch more testing this evening and edit in the results to this post.
EDIT:
Results are in!
From the custom battle testing I did (again, Britannia from the Kingdoms expansion), it looks like the culture determines the layout of the fortress or citadel (and, I would assume other settlement types). I did a NE fortress on two different maps, and both resulted in an identical fortress layout.
For Northern European, Southern European, and Middle Eastern fortresses, siege towers cannot engage the second wall. The game will not let the player even attempt it. They can roll right up to them and certainly get close enough, indicating the hill's slope is not a factor.
I did one test with a Northern European citadel. The towers cannot engage the second or third wall, even though they can move through gaps in the walls. I didn't bother trying Southern European or Middle Eastern.
So it appears to be an issue with the game's code. Again, if it is done that way on purpose or if it is a bug is unclear. I suspect they forgot to include the code to allow siege towers to engage interior walls, but perhaps they did it that way on purpose. Who knows? Either way, I'm pretty convinced it's a code issue.
If there are any modders out there who know anything about how siege equipment engage walls, I'd love to hear your input!
No, I'm not being sarcastic.[This message has been edited by Smileyman007 (edited 12-03-2007 @ 00:40 AM).]