If you do make new siege scenes you really need to enchance them. If you're able to do the ballista thingy for the calradians you should make a catapult or two for the invaders in sieges, and please for the love of dr pepper add more than one entry point. multiple siege towers advancing, ladders being raised, catapults knocking holes in the walls.
There was this one sarradin city that I attacked that had a small door on the opposite side of the wall from the ladder/siegetower. It could be destroyed by hacking at it with your weopon and then you didn't have to use the siege tower to get in. unfortunately the a.i. didn't use it. but still, there should be away to make the sieges more awsome.