double walls sadly don't work in RTW. you can make them and they look pretty (and we did this) but they don't work. This is because the secondary walls do not connect and so where there is a seam (say the gate house connecting to the wall) it is as if there is a hole in the wall. If troops run over these seams they fall to their death. So we decided to pull the secondary walls out of the game. Not to mention they also completely borked the path finding in larger cities. It would appear the game just isn't designed to utilize multiple walls. As for seige weapons, we can't modify siege animations and so a trebuchet is out of the question. However their will be catapults (not those ulgy RTW ornager ones, there will be new models) and ballista.