Every year, the peace rating is updated as follows:
After the rating update, the number of “peaceful years” is increased by one, but when peace has been disturbed this year, “peaceful years” is reset to 0.
Disturbances of the peace are defined as: rioters have been generated or significant buildings have been destroyed in this year.
Significant buildings are all buildings except:
Walls and aqueducts also don't count.