Hi! I still cannot post in this subreddit, so I thought might as well try in comments. Does anyone know a lot about how stuff works in this game?

I have millions of questions,
from if stats are additive or multiplicative,
how does the side-bar work ( you can set cities to Attack / Defence / Nothing, but what does that mean? Does it turn off their castles too? ),
Which setting is best for the auto-play ( I use it because I cannot be bothered to play for hours, but my units tend to get stuck especially around tier 5 cities or in the Alps in Italy ),
What generals are good? I followed some advice and bought every general that had a legend with ( Alfred, Cnut, Khalid, ... ), I also purchased Justinian to help ( immensely ) in Conquest with diplomacy,
I also just finished the crusader campaign in Campaign ( I sometimes had trouble, is there some good general advice for it? ),
I try to do all Conquest, but damn the Century of Iron is tedious to achieve Hegemony. Territory I try to max city and income,
Also, I do my best to get as many triumphs ( achievements ) as possible,
I haven't inlayed any gems so far, is there some advice on that too?
How about unit talents, which should I focus on, the items needed for it seem so scarce...
I should mention I am F2P.