Ok nuff being sarcastic, I shall talk more seriously.
The reason why I posted this suggestion was my observation on how people wished to have some defense mechanism on their castles in
strategus battles. I really cannot foresee how the new wall implementation shall effect the sieges, therefore I will explain my ideas depending on the current wall system.
Firstly, we may want ballista to be less powerful than catapult. I guess everyone can agree on that. Properties may be;
1) the ballista being stationary, therefore having less degree of freedom both in control and localization.
2) could only be deployed on walls with enough space
3) throwing big arrow-like ammo unlike catapults hurling rocks ( much like an oversized siege arbalest )
4) being able to damage only players and player made structures ( siege shields, catapults, ladders etc.)
5) having much shorter range
The other properties could be the same with catapults, like unlimited ammo, being made of dozens of construction material and other stuff.
Assuming that all my enlisted properties are implemented, the attacker side shall still be advantageous over the defenders in means of siege equipment. As ballista crew will be limited to build their weapon only on walls, attackers will have the choice to start their attack on the ballista spot. Also, attackers are already given the chance to relocate their siege equipments if a ballista has been constructed against it. As the ballista is stationery, this will make the defenders to lose one of their ballista in means of being useless after all. Unlike the catapult firing system ( arch-shot ) the ballista will require a clear view in front ( like crossbows ) and this will avoid a complete shield-wall to protect it.
So after all, ballista will not be an indestructible and over powered siege equipment but a weapon of choice for the defenders to have a chance against the incredibly effective catapults.
Like all any improvement or addition to this game, this system shall be criticized and there will be people asking for you to nerf it. But eventually a balance will be brought and we will gain another fun to play equipment in cRPG.
I'm a computer engineer myself and know the means of coding and debugging and all the hours it takes away from your very soul. Therefore I am just suggesting this new feature naively.