Didn't Archimedes use giant catapults in the defense of Syracuse? Unless that's a mistranslation or an outright mistake, it makes me wonder how onagers could be out of period (is there a simpler pre-onager?) But then, these were supposedly pretty big catapults. And the game doesn't seem to allow for any such thing in the defense of cities which is really a shame. Maybe if the onagers weren't so stupid as to nuke their own buildings and walls even when in a good position. :p Ballistas should have been wall-placeable I figure.
And speaking of sieges, those darn wood and pallisade walls usually seem useless. They actually make it easier to archer the enemy to death, unless he withdraws to the center of town, in which case why have the walls anyway? It seems pretty clear archers should have been able to take defensive positions (defensive bonuses vs. missiles) on the lesser walls, to make them worth having.
Or, maybe, archers on the outside of walls shouldn't be able to target enemies on the other side that they couldn't possible even see. Always made me wonder how the invading archers could shoot blind over the walls and still do so well.