Aphrodite Narcissism


My question is how does exactly Aphrodite's Narcissism spell works. It says it makes defending units 10% less efficient in a city. My first question is, does the spell apply to a siege if the spell is used on the sieged city? My second question is how does it apply exactly? Let's say the defending units have 130% defense from advisors and a research. Would the spell take away 10% flat from that making it 120%, or would it take 10% of the overall strength making them 13% weaker?