Sometimes when I bombard a city to reduce its defences, the siege weapons do significantly less damage to the culture defense than they are suppossed to. For example, I went up to a city with a 100% culture defense bonus, and with 6 catapults I only did 14% damage at a time. How do I fix this problem?

Can you post a save game right before one of the bombarding attacks you think is messed up?

I can't recall, but I thought that seige units do a variable amount of damage when they bombard. Like I said though, can't remember exactly.