How about a new promotion available with combat 3 called Engineer. It is avilable to melee and gunpowder units and lets them do 2 things.

1) Rebuilds x% of city defences every turn, even if the city is being bombarded

2) Lets the unit bombard city defences by x%.

It would basicaly represent the engineer corps in modern armies, or underminers in the meideival period.

great idea. I like it. :D

very interesting idea

Yes it always anoyed me that their was no way to counteract bombardment on a Cities defences other then just waiting for it to "heal" on its own.