Multi-city strategy

Once you have a few cities, you have to think of how you want to use them. You could have every city in a balanced state of defense and offense, but there are certainly uses to city specialization.
There are limits to this game that urge you to be effective. You cannot move troops between cities, only support, and there is a maximum population to each city, so your army can only be a certain city. By specializing cities, you can make sure to have the optimum amount of attackers and defenders where you need it and you can get to pick the techs that fit the strategy of each city (which is useful, since it means you don't need to build Library to get all the techs you want)

Offensive strategy

When you attack, it's often beneficial to have a lot of troops of one attack type. So, it would be nice the city that is nearest to your targets could spend all its population on slingers. If you build thermal baths as one of your special buildings, your maximum population once you reach farm level 40 will be 3500. At least 500 population will be tied up in buildings. Ideally, that would leave enough population for you to build 3000 slingers to attack with. However, since you're not attacking on your own island you will need transport ships. Normal transport ships can transport more troops for their population cost, however, since they're very slow, they will normally not be a valid option if you plan to use your city for raids, not just conquests (with conquest, the speed doesn't matter, since colony ship is slower). You will have researched Bunks, which means that each fast transport can carry 16 units. If you have 3000 population available, you would be able to make 2280 slingers and build 142 fast transports to transport them. However, the problem is that if you don't have any attacking ships escorting the transports, they will all die when faces with any ships from the defender. A part of the offensive strategy is to have a city where you build only light ships to use for attack. The light ships can be used to screen for defensive ships so that your conquest army won't face any. When you attack, you can time your attacks from the different cities so that the attacks with the light ships hit just a few seconds before your main attack. Having all your light ships in one city will mean that you only have to time attacks with light ships from that city, not all your cities. Still, you should always send some light ships along with your main army so that are never completely dead if the defender intercepts with defensive ships. With a maxed army worth 3000 population, you could have 40 light ships, 1950 slingers, 121 fast transports.