I am\was aware that the city of Rome would have a few 'issues'...especially in the area of the city where the Colosseum is located. The reasons for this are several. The limitations on me in terms of what I had to work with (just the old RTW files, mainly), and the terrain on that side of the city. When I started working on the cities, I had made some changes in Rome, and they looked good, but a few people said: "Why don't you make Rome look more like Rome did?" Well, ok, that required a lot more work, and presented a big problem because that huge Colosseum had to be moved from why CA put it to the other side of the city....and there was no room for it. So I wandered off into uncharted territory, dug up the old RTW wall editor, and moved the walls on that side of the city 'out' so that I could put the Colosseum over there. That worked ok, but created another problem (which is what usually happens when you screw around with stuff.

) The ground on that side of the city (as bad luck would have it) slopes downward....so my newly created walls ended up being about six feet off the ground on that side at their furtherest extent. This happens because the walls were now 'off' the underlay for the city (which flattens the ground), and there was no way around that except to have a new underlay...which was never going to happen.
My solution to this was to very 'slightly' move the walls 'down' as they went into the slope of the land, and then back up again as they met the sections of wall still sitting on the underlay. It is hardly noticeable, but if you look real close you could probably see it. I was then worried if units could walk on the wall, so I tested this by sieging the city and walking my units along that whole section of wall to see that they could do it without a glitch. And they did, so I figured it was OK.
Now, as I mentioned, there is no underlay or overlay in that area around the Colosseum...therefore, no 'path finding' laid out, so to speak. So units have to make their way as best they can around the buildings over there. I attacked the city several times over there, so I observed the difficulties. But, I just figures: Hey, this was a big city with congested streets and 'close' buildings...it's actually a bit more realistic. But only in that area.
As for the weird behavior of the AI defending army....yeah, that's just RTW's weird behavior sometimes, which is only evident when you wait until the defending army is forced to come out.