I'm posting my findings/experiences in these two areas to try and help others avoid my mistakes.
In my first game (which I'm still playing... stupid job!), I found my military got fairly bored and City-States had resources I wanted... so I attacked them. Actually I attacked and conquered four of them. Just before attacking the fourth, every civ came to me and started calling me "bloodthirsty" and that they "can't share this world with me anymore", etc. Then, after the fourth was conquered, they ALL declared war on me - world vs. me! So be careful and become friends with them - don't just go around annexing them.
Secondly, I discovered a major penalty about losing a previously-annexed city to another civ. Japan's northern army was impossible to fend off (my forces were all in the south and doing well) and did conquer one of my prior-annexed cities (from an old war with a now non-existent civ, Siam) - which proved to be very costly. After I re-conquered it, every building now no longer existed (including the courthouse) and the population dropped from 6 to 1. The courthouse took 40 turns to build, and now I have to build it all over again, suffering major unhappiness all over again. This was tolerable in Civ IV since building was much faster and rush-building via whipping was an option, but now it is very costly.
So, in summary:
1. Don't attack city-states unless doing so to win favor with another city-state. If you do attack them, be wary of seriously hurting relations with all other civs.
2. If you annex a city, then lose that city to a foe, and then finally re-capture it again, all improvements are lost and the population is nearly eliminated. This is true even if the original civ you conquered it from no longer exists!
In my first game (which I'm still playing... stupid job!), I found my military got fairly bored and City-States had resources I wanted... so I attacked them. Actually I attacked and conquered four of them. Just before attacking the fourth, every civ came to me and started calling me "bloodthirsty" and that they "can't share this world with me anymore", etc. Then, after the fourth was conquered, they ALL declared war on me - world vs. me! So be careful and become friends with them - don't just go around annexing them.
Secondly, I discovered a major penalty about losing a previously-annexed city to another civ. Japan's northern army was impossible to fend off (my forces were all in the south and doing well) and did conquer one of my prior-annexed cities (from an old war with a now non-existent civ, Siam) - which proved to be very costly. After I re-conquered it, every building now no longer existed (including the courthouse) and the population dropped from 6 to 1. The courthouse took 40 turns to build, and now I have to build it all over again, suffering major unhappiness all over again. This was tolerable in Civ IV since building was much faster and rush-building via whipping was an option, but now it is very costly.
So, in summary:
1. Don't attack city-states unless doing so to win favor with another city-state. If you do attack them, be wary of seriously hurting relations with all other civs.
2. If you annex a city, then lose that city to a foe, and then finally re-capture it again, all improvements are lost and the population is nearly eliminated. This is true even if the original civ you conquered it from no longer exists!