There is only 1 way I know of to take the province without damaging the strucures and that is to bribe the besieged garrison. This makes it so that you are relieving a siege and no damage will be done. I remember trying to bribe some Italians, the genral had the vice that gives minus 50 percent to bribery cost and could'nt assault it as the loss of troops would have not allowed me to hold that province and it was going to hold out for 2 years and I did'nt want the Italians to have an opportunity to releive the siege. Unfortanately he was to loyal and chose to starve to death.