I was told the following:-
In order to stop the CS you just have to have one extra troop left after he attacks you. So if you have 4 cities around it. Then reinforce your city with all of your other cities. Therefore hopefully you have more troops then him and you will keep your city. Also remember to have your militia from the farms join the fight
If i support the city with more troops than he has sent, will i succeed or fail ?
Not sure because he has a lot more ships than me & others in his alliance have sent him biremes & LS as support :/