Baaad ideas. Civ IV is too defense oriented already. It takes literally hundreds of years to lay seige to a decently sized city. Making it take even longer is just silly.
The attacker already pays dearly for assaulting cities. Archers get a standard city defense bonus, plus the cultural defense bonus for the culture level of the city, plus the bonus for walls which the AI is likely to build, plus city garrison promotions which AI also has a habit of bestowing upon archers, plus the stack aid bonuses that come from AI's tendancy to put 3-6 units in a city.
Maximum of 5-10%? That is just ludicrous. You already need to out do your enemy about 4 to 1 to take a city, plus the enemy heals faster because they're in a city.
A catapult striking a city walls may not do much. But we're talking about a battery of catapults being fired repeatedly over many years. Around the time of catapults, 1 turn = about 10-20 years. If you had a few catapults and enough ammo, the average city would fall in a week or two, and you think it should take longer.
Make the maximum damage less than 50%. Definately not. If you bombard someone repeatedly, a magic shield does not appear around them once they get injured. Throw rocks at someone repeatedly and, make no mistake, they will die.
The max damage should be all the way at 100%. Now that's realism. Maybe doing less damage to units per turn though. In my curent game, I seem to be killing units with one bombardment.
There should be a downside to mega bombardment though, like destruction of buildings (barracks, library etc.) and severe population drops. Meaning that if you pound a city into dust, you won't benefit from claiming it afterwards.
WarKirby
) and most of the time they fall after a siege of an average of 6 monthes. The longest one i know is four years. It's long but this English Castle was too close to Paris ( around 60 kilometers) 
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